What isn't written in the description, is written here. What was originally a thread post, I decided to move to a guide so that players can find it much more easily.

I believe that most of the card placements on this guide/tier list are accurate to most user's experience with the game, with the exception of cards like Steal and Crusade, being a guaranteed S rank in most people's eyes.

Again, do take this guide/tier list with a grain of salt; this was written with my experience playing Yggdra Union, and my experience will most likely differ from yours, the reader's experience.

Also, some of the card images are larger than others, and I didn't resize them, so please don't mind that.

This list/"guide" contains quite a few spoilers, so if you don't want anything spoiled, try not to read up into cards that you haven't obtained. Some descriptions for cards also contain spoilers even if you obtain them early on.

In regards to the criteria in which I based this list on:
Positive Factors:
+ Anyone can use it
+ ALL ACE
+ High MOV
+ Status Effect
+ Effectiveness in clashes
Negative Factors:
- Only one character can use it(Steal, Chariot, etc)
- Low MOV
- Essentially useless
- Requires specific weapon for ACE
Neutral/No effect on rating:
* Base Power when the card is acquired
* When the card is received

Anyway, without further ado...

MOV: 9
ACE: Axe
Type: Break
Conditions: None
This is a no-brainer. High Move(9), best status effect(CURSE) in the game that very few enemies are immune to, Absolutely DESTROYS Valkyries. Makes 99% of the fights in the game laughably easy due to CURSE neutering enemy attack speed and damage. Anyone can use it as long as Milanor or anyone equipped with ALL ACE items leads the union. Status effect also increases morale damage dealt to the enemy.

Curse also has the added effect of inflicting Slow, but on steroids: It makes victims only able to move one tile, while decreasing their attack power and attack speed in half. You CAN still lose/win with Curse status inflicted on you or the enemy, but it's highly unlikely.

Anyone who has used Gravity Chaos can and will tell you that it is hands-down the best card in Yggdra Union

S Tier where cards that are widely used among many runs, and are commonly accepted to be universally useful in all, or most points of the game that they can actually be used.

MOV: 6
ACE: ALL
Type: Active
Conditions: None
Decent move, protects from any and all attack damage, including fatal ones, excluding very specific attacks from certain bosses. I don't remember if it protects against status effects or not. ALL Ace card, meaning anyone can lead the union. High TEC stat also makes you start the battle as high as 90% meter with this card. Becomes ridiculously overpowered when used on a character equipped with an item that has 2x Skill Time. You can also just activate shield barrier right before the enemy activates their card, to preserve the command gauge; as long as the shield is active, it will block the attack.

MOV: 8
ACE: Sword
Type: Active
Conditions: None
Banshee's Cry is basically Gravity Chaos but mildly weaker in every aspect. Less move, but reduces enemy attack to 1, which is arguably as strong as, if not stronger than Gravity Chaos in the late game, when enemy attack averages to 4 stars. Has immense effect when used against bosses, and especially lethal when paired with Gravity Chaos. Sword ACE card, so only Yggdra and Russell can lead the union, or anyone equipped with an ALL ACE item. TEC stat has no bearing on the effectiveness of this card although high TEC stat also makes you start the battle as high as 90% meter with this card. Becomes ridiculously overpowered when used on a character equipped with an item that has 2x Skill Time.

MOV: 6
ACE: All
Type: Break
Conditions: Same Gender(Fencers cannot convert Valkyries), Same Size(Knights cannot convert Bandits/Fencers and vice-versa)
Decent move, abducts enemy units if the units are the same size and gender, can otherwise turn a losing battle into a winning one if the user's TEC is high enough. ALL ACE card, meaning anyone can lead the union. Especially hilarious when fighting Nessiah. Also makes Aegina and Luciana hilariously easy when fighting them with basically any female character(because for some reason, all female characters just have innate advantage over Valkyries...). Try to use Mind Change when your units are 4 or less, and the enemy has 4 or more on their side. 1 star of TEC over the enemy=2 Units(or 1 if Knight), Less TEC than Enemy GEN means the skill will fail. You can only convert as many units that are missing: if you are missing 2 units, you can only convert 2 enemy units, which makes it more of a counterplay card than an offensive one. Fantastic if the player's character(s) have low(2-3) GEN, as enemy counterattacks will make the player use more units, thus making Mind Change much stronger.

MOV: 9
ACE: Bow
Type: Break
Conditions: Assassin Only, Night Only.
Even though it's a Night-only and Assassin-only(basically Elena) card, the fact that all the late game maps are locked to Night makes it top-tier, and the reason why Elena can 1v5 unions. Kills the enemy head(with the exception of Marietta, No.367) as long as TEC is higher than GEN... and Assassins have incredible TEC earlier on. Also has 9 move, so it's a fantastic card in general. Killing the enemy head also means that they cannot activate their card effect at max RAGE meter, allowing you to prepare half of the command gauge by going passive to start next round with an immediate use of Bloody Claw, since the meter starts at around 50% at max TEC.

A Tier is mostly comprised of cards that are really good, or fantastic, but because of their restrictions, don't quite make the cut into S tier. Yes, this includes Steal and Crusade, both of which are highly regarded as two of the best cards you can use, due to their utility/power and high movement. However, as stated, due to the nature of being locked to one character, they didn't make the S tier cut.

MOV: 12, the highest of all cards.
ACE: Axe
Type: Break
Conditions: Milanor Only
Before you all get mad at me for putting everyone's favorite card on here, hear me out. Steal is a Milanor-only card, and if his TEC is less than enemy GEN(rare, I know), the card will fail to steal the item. Not only that, there are some situations where stealing the enemy's item made them harder to defeat, because the item reduced their stats by a considerable amount. Stealing(or having an enemy's item drop due to higher LUK) cycles out the item, and makes an enemy have a new item to drop/steal, and there are very late game maps where the item that replaces the previously dropped/stolen one can make certain fights incredibly more difficult.
In reality, the only redeeming factor to Steal is the fact that it has 12 Move, which isn't really a good enough reason to put it into S tier. Steal is still a fantastic card despite all of this, however, and don't let this discourage you from using it; it's usually most player's first card that reaches 9999 power.

MOV: 7
ACE: Rod/Staff
Type: Break
Conditions: None
Status: Burn
I was debating putting Flame in B tier because reducing enemy morale via the damage over turns reduces power gain, but the loss is marginal. The only reason I put Flame in A Tier is because you get Grilled Griffon and Grilled Dragon when killing a Griffon Rider or Dragon Knight with this card, which both give +1 GEN to whichever character eats it-- tremendous for characters like Mistel whom have pitiful GEN when you first get them. Flame also deletes Undines from the face of the planet. Flame also has decent move rating. Status effect also increases morale damage dealt to the enemy.

Becomes stronger when used on a character that enhances Fire damage, or equipped with fire enhancing items. If used on Rosary equipped with a Ruby Staff, Flame becomes an Fatal Damage/instant win.

MOV: 7
ACE: Bow
Type: Break
Conditions: None
Status(PSP+): Paralysis
Thunderbolt is what you use when any of the cards in S tier is unavailable, or if you really just want enemies to not move before/after using Gravity Chaos. It's basically Gravity Chaos, but requires Cruz/Elena/ALL ACE to lead the union, with a weaker invisible debuff to stats when the status is on your units or the enemy. Good move makes it a generally good choice, once you have Elena, because Cruz is laughably useless in the late/endgame. Status effect also increases morale damage dealt to the enemy. Becomes stronger when character(s) are equipped with Lightning-enhancing items.

Keep in mind, Paralysis prevents characters afflicted with it from moving, so do exercise caution around enemies that can use it, and organize your unions in a way that their GEN is high enough to not be paralyzed by the enemy's use of Thunderbolt.

MOV: 10
ACE: Scythe
Type: Break
Conditions: Tactician/Mistel Only
Additional Note: Cannot be obtained in the GBA version, alongside the character "Mistel".
Oblivious Dawn is a card that is restricted Mistel Only. It has fantastic move, equal to Revolution. Oblivious Dawn, when activated, reduces enemy RAGE, but if Mistel is weaker than the enemy, she'll still be likely lose... and it takes a TON of investment to make Mistel strong. Has no real downsides if you can actually make Mistel strong, but Banshee's Cry is basically Oblivious Dawn, with very little restriction. Still a great card overall, but terrible/useless if you don't use Mistel.

MOV: 4
ACE: All
Type: Break
Conditions: Only Union Leader can activate
Being an ALL ACE card, Ace Guard can be incredibly helpful in many situations, and truly benefits strong characters that are fighting in two or more battles at once without a "Negate Battle Penalty" item equipped. Only the union leader can use it, but once the card is active, it prevents Counterattacks and Charges from damaging your team.Protecting against counterattacks is incredibly useful on any unit except for Elena/Cruz(on charges, great if being attacked). Don't use it if you regularly utilize Pamela+Coma Karma. The only real downside to Ace Guard is its 4 MOV, which is among the lowest MOV in the game, stopping it from bumping up to S Tier.

MOV: 9
ACE: All
Type: Break
Conditions: Yggdra only, Head is alone(all units in current battle are left to 1)
Yggdra ONLY. Requires Yggdra to be alone(we'll never fight alone, amirite?) to activate, meaning that the 7 valkyries with her must be dead to activate Crusade. Once activated, you instantly win the fight, though you have to activate the skill before the enemy power attacks or interrupts the charge with skill activation. Defeating an enemy with Crusade deals bonus Morale Damage to the enemy. Probably the second or third card most players will achieve 9999 power with.

Extra Note:Crusade is also required to slay Marietta when she has 1 Morale remaining for the Ragnarok Ending(normal difficulty), and to proceed to the Realm of the Gods on Hard mode after slaying Marietta with the card.

MOV: 8
ACE: Sword
Type: Break
Condition: Fencers only
Effect: Disables target's equipped item if TEC is greater than target's GEN
Russell only. Originally placed in C tier, recent information has brought to light that Item Break no longer destroys the item(It used to in GBA, not sure about PSP), but disables its stats/use. From that alone, it gets bumped to A tier, as it's just a faster-charging Steal, in essence. With High LUK, you can still easily earn the item, or just steal the item on the following turn(assuming the enemy is still alive).

B Tier is the tier of "Sometimes it's useful, but I only just use it for moving most of the time" tier. The cards here are good, but only under certain cases. With the exception of Blizzard being good only when used on Nietzsche equipped with Ice Javelin, most of these cards don't have much practical application, especially compared to things like Gravity Chaos, Banshee's Cry, and Shield Barrier. Feel free to bring and use these, however. Just because they're not the best, doesn't mean they're the worst.

MOV: 7
ACE: Spear
Type: Break
Conditions: None
Element Damage: Ice
Effect: Freezes Water Tiles until the next NOON Phase when used by Undines/Nietzsche. Ice Never melts on time-locked stages.
Additional Effect 1: Incredibly Strong/Fatal Damage to Dragon Knights.
Blizzard, or more specifically Diamond Dust is great for screwing with enemy undines by freezing the water when activating Blizzard using Nietzsche-- it removes their GEO Advantage, and turns it into a penalty, on top of creating movable terrain until NOON the next day. If a unit is on a frozen tile during the next NOON, the ice will thaw, and the unit basically dies from being unable to swim. Decent move, Spear ACE(Durant, Nietzsche, ALL ACE item). Somewhat useless in later maps, but still incredibly strong when a character is equipped with Ice-enhancing items. Becomes Fatal damage when used by Nietzsche equipped with Ice Javelin-- incredibly overpowered against Dragons late-game. All in all a decent card, but doesn't quite make the cut into A tier due to the card being locked to two mediocre characters(Durant, Nietzsche). Durant and Nietzsche are really only useful on stages with lots of roads/bridges and water. Durant becomes useless in the forest sections of the game, and Nietzsche becomes near-useless in the desert stages of the game

MOV: 8
ACE: All
Type: Break
Conditions: Noon Only
Effect: Revives fallen allies(1 TEC=1 unit)
I thought about bumping Sanctuary up to A tier, much like mirage, but the practical application for Sanctuary isn't as good as Mirage. Being locked to Noon only makes Sanctuary useless in the later parts of the game, when time is locked to Evening or Night. Good move, and great for a lot of the earlier points in the game, but just falls off hard. Until the late point of the game, and certain maps that aren't able to reach NOON, Sanctuary is still a great/useful card. Don't invest too hard in this one, and use it more as a backup card.

MOV: 11
ACE: Rod/Staff
Type: Break
Conditions: Necromancer Only, Witch Only
Effect: Summons Skeleton/Golem, deals damage
I don't really see a reason to use Necro Gate or Doll Craft other than to peel enemies with skeletons, or destroy bridges with golems. Requires Roswell/Rosary, and can only be used by them. Decent damage, fantastic move, not very useful in the long run. Not much else to say, really.

MOV: 6
ACE: All
Type: Break
Effect: Switches ATK and LUK stats: 1 LUK>1 ATK
Makes Pamela and Cruz useful due to their LUK being disproportionately higher than their ATK at base, but otherwise isn't very useful. Decent move, ALL ACE card. No reason to use this when Gravity Chaos and Banshee's Cry exists.

MOV: 8
ACE: Axe(GBA), All
Type: Break
Effect: Raises combat power against Dragon Knights to Red Star rating. +50% Morale Damage to Imperial Knights upon victory.
Dragon Killer just... makes you fight at red star advantage against dragon units, but doesn't guarantee a win against them. Still a fantastic card that makes the late game fights much easier. Useless otherwise. If you've invested on Nietzsche or Durant, just use Blizzard over Dragon Killer. If you need to, pair Dragon Killer with either Gravity Chaos or Thunderbolt: after you've inflicted either Curse or Paralysis on the enemy, Dragon Killer will essentially make you an unstoppable force against them.

MOV: 10
ACE: Sword
Type: Break
Conditions: Yggdra/Valkyries Only, Head is alone.
I considered placing Revolution in C tier, but 10 move saved it. Yggdra/Valkyrie only, and is useless outside of its 10 move once you get Crusade. No other application otherwise. Also doesn't guarantee a win unless Yggdra has higher ATK than the enemy unit, and has neutral or good advantage against the target(she'll likely still lose to Knights or Undines). Also becomes useless when you lose Yggdra and have to rescue her.

MOV: 3(lowest in the game)
ACE: All
Type: Break
Conditions: Noon Only
Effect: Heals Status Ailments, Restores some Morale(not in GBA version)
Refreshment has the lowest move among all the cards in the game. Its only real use is free Morale Heals for a turn, or if you get slapped by Gravity Chaos/Thunderbolt on certain maps. Otherwise basically useless in all aspects. ALL ACE card. Becomes useless on maps that are locked to non-Noon phase.

MOV: 7
ACE: Sword
Type: Break
Conditions: None
Element Damage: Holy
Banish would be in A tier if not for the fact that a huge amount of the game's primary enemies are Valkyries, making Banish less effective than the other element cards, and what makes Gravity Chaos so godly. Sword ACE, decent move, Yggdra's giant middle finger to anything weak to light element. Useful against skeletons and necromancers(excluding the priest necromancers in the desert), decent attack card otherwise. No real complaints on this, but not a go-to attack card like Gravity Chaos, Flame, or Thunderbolt.

MOV: 6
ACE: All
Type: Break
Conditions: Night Only
Effect: Empowers player character(s) and weakens enemy character(s) for 7.77 seconds in battle. The head loses automatically if the battle is not won during the 7.77 second duration. Prevents enemy from activating card during Kiss of Death's active time.
All ACE card, decent move, Night only, don't use it unless you have high REP, and the enemy has less than half their units remaining, as the head dies if the fight lasts more than 7.77 seconds. Very niche use, but powerful when used properly. Generally is not used until Gravity Chaos, Banshee's Cry, Shield Barrier have all been used.

C Tier... basically the dump card tier. The kinds of cards that will almost never see use outside of being a last resort. The effects range from awful to okay in certain situations, but never truly be considered as "Good". This tier specifically is not ordered from best C tier card, to worst, as they are all situationally useful.

MOV: 4
ACE: All
Type: Break
Conditions: Noon only.
Highly underestimated card, due to low MOV and Niche effect, but you can create some jank combinations with it. Ever have any issues with an enemy that's standing on a fort or a geo tile that gives them the advantage in combat? Is the enemy standing on regular ground while you're standing on Swamp or Ice? Mirage swaps the tile for you, and also removes the GEO Morale Defense on winning the battle for the enemy. The things that draw this card back is that it has very low MOV, and is locked to NOON-only, making it useless for late/endgame maps, which are typically locked to Evening/Night. If not for these drawbacks, it would easily make it into A tier, if not S tier.

MOV: 7
ACE: All
Type: Break
Condition: Pamela Only
Effect: Puts target to sleep.
Honestly a hilarious card, I debated putting Coma Karma in B tier, or even A tier, but being locked to Pamela really hurts Coma Karma as a whole. Pamela isn't bad, but she just isn't Rosary or Roswell. When you put enemies to sleep, they basically become unable to act, leading to perfect victory when you attack the enemy and its union, which results in much higher Morale damage(especially if you crit). If you don't use Pamela, this card is useless outside of decent MOV.

MOV: 9
ACE: Bow
Type: Break
Condition: None
Effect: Target hit turns to stone if TEC is higher than target's GEN
A very stupid card that turns the victim to stone. Turning a victim to stone makes them unable to act or move, and suffer no morale damage. They also become a physical obstruction, which can greatly affect how you approach a map. Medusa Eye has no real use outside of its high MOV, or if you just don't want to deal with an enemy anymore.

MOV: 5
ACE: All
Type: Break
Condition: Enemy is in Wasteland tile.
All ACE card, low move, niche use. Requires the enemy to be in a wasteland to use it, and inflicts Slow to the target if their GEN is lower than the user's TEC. Not much use otherwise besides dealing damage. Very annoying on maps where the enemy can use it, though. Only got promoted to C tier due to its status effect being somewhat useful(but infinitely worse than Gravity Chaos and Thunderbolt)

We have finally arrived to the "What is the point of this card?" tier. These cards are basically useless for the majority of the time you own the card. They might still see some use if you're trying to do some gimmicks or want to focus resources into Durant for once.

MOV: 10
ACE: Object
Type: Break
Conditions: Skeleton/Golem Only
Effect: Inflicts Poison if TEC is greater than enemy GEN, Deals damage, breaks nearby bridges and structures.
Skeletons/Golems only, Earthquake destroys nearby tiles without sacrificing the golem, and Poison Breath can inflict poison on the enemy if the skeleton has higher TEC than the enemy's GEN, but otherwise... both are useless other than their very high MOV.

MOV: 9
ACE: Spear
Type: Break
Conditions: Knight Only
Effect: Reduces enemy numbers equal to the user's remaining units.
Durant Only, basically useless as it requires Durant to have higher ATK than the enemy to win, so you really only want to use this if Durant is on a Road or Bridge. Good move, but otherwise little to no use in practical application. Truly a bottom tier card, even if you pour your resources into Durant.

To clarify, despite what is written above, Chariot is a pretty solid card, it just requires a few conditions to actually make it work really well/win.
- Durant must be within 1.5 or 2 Large ATK Stars against Fencers and Valkyries, any less and he will still lose. Equal ATK or within ~1 large ATK star means he will always win against Fencers/Valkyries.
- Durant must be 4 small ATK stars stronger than the target at minimum(as a general rule of thumb)
- Durant must be 1 Large ATK Star stronger than the target if the target is a Knight, or Bandit/Axe User, or Mages.
- Durant must be 1.5~2 Large ATK Stars stronger than the target if the target is a Griffon Rider, or Imperial/Dragon Knight
- If Durant is on a Road or Bridge GEO Tile, divide the required amount of stars by half-- he will still most likely lose to Gulcasa, however.

Chariot reduces the enemy to be the same number as Durant's unit-- but Durant still needs higher ATK to win the battle against an enemy(not counting sword users). You can either use Chariot to stall/lower morale damage dealt to Durant, or pull out a win by a hair's strand.

Basically the most useless cards in the game, where they are only usable in the specific chapters you get them in, excluding Rockfall(C Tier), since there's wasteland tiles in a lot of maps. Otherwise only used for their pitiful movement. I, and most other players would never take these otherwise. I'm not going to include any information about them except for their Movement, because I know anyone who reads this will likely never use them unless they absolutely have to. The description I would have otherwise provided can be found in this overview of this tier, except reworded to fit either Forest, Swamp, or Sand.

MOV: 5
ACE: All
Conditions: Enemy is in Forest tile.
Additional Effect: Deals fatal damage to Knights

MOV: 5
ACE: All
Conditions: Target is in Swamp Tile.

MOV: 5
ACE: All
Conditions: Enemy is in Sand Tile.
Additional Effect: Fatal to Undines.

So that's it, my personal rating for every usable card in Yggdra Union. I'm sure most people can agree with most of the ratings for every card, minus certain ones like Steal, Crusade, Mirage, Sanctuary, and maybe one of the elemental attack cards.

I'd make a tier list on characters, but to spare the effort of writing another guide, let's have a mini tier list right here:

Yggdra- Crusade, Revolution, Banish,
Milanor- Steal, Gravity Chaos, Wasteland Advantage(great for Hard Mode's post-game area)
Elena- God tier, especially with Bloody Claw in the late stages of the game due to Night-locked.
Roswell- Dominates Valkyries, which are very common enemies, has innate advantage against every weapon type except bow. Access to Flame.

Rosary- Flame damage is extra powerful, Good against Gulcasa due to Null Fire element
Nietzsche- Diamond Dust absolutely destroys Dragon Knights
Mistel- Terrible until heavily invested, Good once average stats are 4 or higher.
Russell- Discount Yggdra, great battle theme, allows you to use Banshee's Cry when you don't have Yggdra. Item Break is also amazing in Switch/Mobile/PC version.

Durant- Good on a lot of maps with roads and bridges, or maps with a lot of Fencers/Valkyries. Bad for everything else.
Gordon- Same as Durant, but temporary character.

Pamela- Discount Rosary, with access to Coma Karma, downgrade compared to Rosary in every other aspect.
Kylier- Discount Milanor with Durant properties. Temporary Character. No point in using her, plus she has a dumb death.

Cruz- Cool BGM, trash when you get him, trash when you get Elena, completely falls off later on when stages are locked to Evening/Night. Only other Niche is dealing extra Morale Damage to Marietta due to Marietta being unable to counter attack(and will likely get zapped by Angelic Thunder as a result).

Anyway, that's all from me. Thanks for reading, and if you have any questions or anything to discuss, please leave a comment.

Thanks to the DeptHeaven wiki for having the images for the cards available.

Errors:
1- Fixed Grilled Griffon+Dragon from ATK > GEN.
2- Fixed parts stating that GEN determines how full the command gauge is at the start of a clash: it's actually TEC.
3- Removed Elena as an eligible candidate that can consume Grilled Griffon/Dragon.

Additions:
1- Added more details/elaborated/clarified more upon the effect of Chariot, and the bearing ATK has on the card/after the card's activation.
2- Added a changelog to the credits section...
3- Added reasons/metric on what determines a card's placement in Section 1(Overview).
4- Included +50% morale damage to Imperial Knights when you win the battle with Dragon Killer active.
5- Added Kiss of Death's prevention of card activation to its effect.
6- Added extra info on Ivy Whip dealing fatal damage to knights.

Card Tier Up/Down
1- Mirage moved from A tier to C tier due to being a NOON-locked.
2- Item Break moved from C tier to A tier due to discovering that it disables the item, rather than destroy it completely.
3- Moved Russell from C tier to A tier in light of the Item Break change.

Future additions:
1- Probably just the base power of each card, and maybe when the player receives them.

Most of what was written here was how I remember it in the PSP version from around a decade ago, so thank you to those who pointed out errors in the guide, and thanks for reading this pointlessly lengthy tier list!

If there are any other changes that need to be made, please let me know!