This thread is supposed to be a brief general guide and a collection of powergaming info, tips, strategies, etc primarily outlining a STR/AGI build designed to dish out as much punishment as virtually possible.

Feel free to contribute any useful information in the comments.

I'm assuming 149% (maximum) difficulty and that the player intends to create his own faction and rule the entire kingdom.

First, a quick examination of attributes is in order.
Skill Influences:
STR influences the most important skills in the game, namely ironflesh and power draw/strike.
AGI influences a couple less important but still essential skills, namely riding, horse archery, athletics, etc.
INT influences party skills that can be instead be assigned to a companion and are an utter waste for your character to take.
CHA influences slightly useful though minimally beneficial skills that you can do without (persuasion, leadership) since they can be accomplished through other means (disposition, renown).
Passive Benefits:
STR provides 1 hp per level, which is absolutely essential to survival (essentially 1/2 an ironflesh rank).
AGI increases attack speed by 0.5% (higher dps with all weapons, particularly the bow) and provides 5 bonus weapon proficiency points (negligible, since your proficiencies will mostly increase on their own).
INT provides a skill point. Taking a a point in INT is essentially trading an attribute point for a skill point. This can be temping in early game but by mid-late game you'll have more than enough skill points to go around and thus there is no reason to make this trade. Higher INT also allows you to read a couple skill books but it is still FAR from worth the sacrifice.
CHA increases party size by 1. The same thing can be accomplished with renown for free.

The best way to look at character creation is to prioritize your attributes/skills and then choose the options that provide the best statistics.
For more info, check here: http://strategywiki.org/wiki/Mount%26Blade/Character_creation
First of all, attributes will be prioritized above skills since by end game you will have more than enough skill points to distribute freely due to the 3:1 attribute skill ratio.
Attribute skill priority: STR, AGI, CHA, INT
For the purposes of character creation choices (father, early life, etc) STR and AGI should be treated equally. CHA and INT should be avoided at all costs throughout the entire game.
I will separate skills into three groups: desired, undesired, and useless.
Desired: Ironflesh, Power Draw, Power Strike, Riding, Horse Archery, Shield, Athletics.
Undesired: All remaining personal skills and all leader skills.
Useless: All party skills.
Recommended character creation choices:
Father: Thief [+3 AGI, +3 Desired Skills, +1 Undesired Skills, +1 Useless Skills]
Early Life: Steppe Child [+1 STR, +1 AGI, +1 Desired Skills, +1 Undesired Skills]
Adulthood: Squire [+1 STR, +1 AGI, +2 Desired Skills, +2 Undesired Skills]
Reason for Adventuring: Revenge [+2 STR, +1 Desired Skills]
With the remaining points you are permitted to freely distribute I recommend raising STR and AGI to 12 (remember the 3:1 ratio) or dumping all your points into STR, putting one point into Riding (to raise it to three for the spirited courser requirement) and dumping the remaining points into Iron Flesh and then Power Strike. These two skills are very important in the early game (though later on Power Draw will become more important than Power Strike).
Distribute weapon points between One Handed Weapons and Archery proficiencies however you like. Your proficiencies will mostly increase on their own.
Choices like father: veteran warrior and early life: page will ease the difficulty of early game since they provide a good bit of your most desired skills (power strike and iron flesh) as well as some undesired skills that are particularly useful in early game (persuasion and leadership) along with their charisma. However, for the purposes of this guide, I would not recommend these choices since powergaming is targeted at end game, not early game, and in end game the CHA bonuses and CHA skills will be nearly useless while you'll always be hurting for more STR/AGI.

Never put any points in INT or CHA. INT is very tempting because of the skill point bonus each point provides. However, in the long run you'll have more than enough skill points to distribute by leveling alone and taking a point in INT is essentially trading an attribute point for a skill point. Because of the 3:1 ratio, you're going to need attribute points a lot more than skill points. There's no reason to trade a valuable attribute point for a skill point. As for CHA (which notably influences leadership and persuasion), your party size limit will increase proportionally to your renown (slowly but for free). Persuasion is a luxury that you can do without. It's nice for a few early quests you'll be forced to take in the early game but in late game you'll only be taking lords into your faction who already have a disposition of 100 and thus persuasion is a very small factor in their decision to join you.
Increase your Attributes by multiples of three. Remember the ratio.
Put most of your points in STR. Ironflesh, Power Draw, and Power Strike are your main skills. The higher you can get them, the better. On top of that, each STR point gives +1 HP. Power Draw will become more important than Power Strike mid-late game. I recommend always keeping Ironflesh as high as possible. HP is obviously a high priority.
Stop (pause) putting points into AGI at 15. At level 5 riding you'll be able to use any horse in the game. Your Shooting From Horseback and Shield skill should also be very sufficient at this level. Keep in mind AGI increases your attack speed. More AGI = more kills per second with the bow when defending/taking castles (not to mention this helps a lot in melee fights). I think there's an argument to be made for pushing AGI past 15 for the attack speed alone once you've reached a very high level. With level 6 Power Draw you can kill just about any unit in the game with a headshot so if you have perfect aim and a lot of skill AGI could be a higher priority than STR. With incredibly high Riding (faster horse speed), horse archery, and shield skills as well, you would be very deadly on the field as well. Once all your STR skills are maxed and all your AGI skills are at or nearly 5 (or at least the AGI skills you find most useful -- likely riding and horse archery) you obviously ought to continue increasing AGI.
In order of priority: Ironflesh, Power Draw, Power Strike, Riding, Horse Archery, Shield/Athletics. Every skill not listed is undesirable if not entirely useless. Pick two companions to be pure INT monkeys and assign them party skills (In order of priority: Surgery, Engineering, Path Finding, Wound Treatment, Spotting, First Aid, Tactics, Trading). It's best to put all medical-type skills on one companion and assign him to his own group which you should order to hold position to avoid combat at the beginning of each battle (if your medic is knocked out then his skills will no longer apply). Any extra companion skill points (especially on pure INT companions) should be invested into trainer. Trainer is not technically a party skill. It is an individual skill that effects the entire party (and stacks).

The Bow is by far the best weapon in the game:
Extremely high damage: Even with medium levels of Power Draw (usually by around level 5-6) all low-tier enemies will die in one body shot and almost anything will go down with a headshot. By level 10, only the toughest most heavily armoured enemies will require more than a single body shot and headshots will do more than any weapon in the game.
Extremely High Accuracy: The most accurate weapon in the game. If you can see it, you can kill it (unless they have a tower shield).
Extremely Fast: The fastest ranged weapon in the game by far. Keep in mind that when defending/taking castles almost every arrow is likely to be a kill. The defense scenario is the most simple. Just find a good vantage point and wreak havoc upon the troops climbing the ladders (go for headshots on the high-tier enemies first). Most good bows can average about 2 kills per 3-3.5 seconds and this will increase with higher AGI.
Between these three factors, the bow objectively has the fastest DPS of any weapon in the game. The only limiting factor if your own aim.
You don't need to bring melee weapons to a castle, but you might want to bring them to the field. You can always pick them up at castle events. I recommend picking up a broadsword or a sabre. There should be plenty of cover so you shouldn't need a shield either (although you can always pick one up, as well). You may drop melee all together by the time your Power Draw is strong enough.
Whether or not you use a shield is up to you. In the start-mid game you're probably going to need one but later on you might find it's obstruction of your view and waste of a potential quiver spot. If you're not good at manual blocking it may be safe to keep one around in case you're thrown from your mount. You're only going to want to bring a bow and three quivers to castle defenses/attacks so if you need a shield or melee weapon in that sort of situation just pick one up. If you're on horseback it's really up to you.
Remember, pressing R will put you in first person mode and allow for easier shots. On horseback, it's probably best to play from third person, however. Note that in third person you'll have to aim slightly higher than you would have to in first person. Try to get used to both aiming systems (first person on foot and third person on horseback).

The first thing you should do when the game begins is ride around and collect all the companions you plan to use in your party before you recruit a single non-hero unit into your group (do note, however, that you will have to slaughter some looters and pawn their gear for chump change along the way in order to afford recruiting many of your members). This way you may maximize XP gain of your companions who will be a significant key to your success. I recommend fighting small groups of bandits with only your companions for a while in order to ensure that every last bit of XP goes to them or yourself and then working as a mercenary. Once you and your companions are strong enough, start raising an army (and your renown). Once you have an army, you need to take a castle.
I do not recommend swearing vassalage to any faction. The primary function of swearing vassalage is to begin fighting large scale battles against high level enemies for the following reasons: To gain XP for yourself and your companions, to raise your renown, to make some money, and to improve your relations with other lords. All of these functions can be satisfied by working as a mercenary. You won't ever be granted a fief, but that's a small matter when you soon intend to rule the entire realm. When you defect as a vassal the relations of your previous faction stick to your character. This can make taking your first castle much harder. Don't even think of defecting while you're at war with more than one other faction (best to defect during peace time) and note that limiting your choice of first castle will be much more difficult when limited to a single faction (and with them chasing you around the realm constantly). When your mercenary contract expires, all negative relations are reset. With that said, vassalage affords you a few interesting opportunities. Most notably, if you are the primary force in the sacking of a castle and the king neglects to grant it to you, you may often call foul play and defect with all your men and your fief. This scenario is, however, quite rare and rarely well timed. You also have the opportunity before leaving your faction to strip the defenses of one of your fiefs so that it may soon be an easy target for your warband. This is perhaps the most valuable benefit of vassalage (although slaughtering your own men in such a fashion ought to penalize your honour). The choice is yours, though I would still recommend you simply work as a mercenary and follow the next section in order to gain your first fief.
Taking your first castle is all about opportunity. Your biggest issue at this point in the game is keeping your troops' morale up while you wait for a good opportunity. The best time to take a castle is just after it is sacked and the victor has sustained heavy losses. Often you'll find a freshly sacked castle with but ~60 troops to hold its battlements. There is, however, more to consider than timing alone. You need to choose castle that is easily defended (this will of course make taking the castle more difficult but if your timing is good you should be able to take it anyway). The best sort of first castle to take is, luckily, one of the most common sort: a single ladder leading to a very narrow way between two tall battlements with a good vantage spot for you to pick off enemies from.
The best way to make friends is to beat lords in battle and honourably let them go. After pounding on a faction for a good while most of its members will be ready to switch sides.
Never accept a lord into your faction that doesn't already like you (I recommend not letting a lord into your faction until they have 100 disposition with you). They will cause problems otherwise.
In case you didn't know, holding ctrl + space speeds up map travel (it's also a great way to get into trouble if you're not careful.

At a siege bring a bow and three quivers and immediately run ahead of your men and pick off all archers/crossbowmen you can see (you might even want to order your men to hold their position if they're not already in the line of fire but beware, with enough enemies aiming at you you might be surprised to find a bolt between your eyes and the siege ended before it had even begun). This can be difficult to do if you're not quite good at aiming yet. The trick is simply to see how far away they are and use that information to determine how high above the little specks of their heads you have to aim. After some practice, this will come naturally to you. Once you've done that, it's best if you can find a spot where you can see their reinforcements though the battlements and spray arrows at them. It's best to avoid going up the ladder/siege tower if you can unless your men have already broken through the front lines. At maximum difficulty approaching the entry point of a castle is essentially throwing yourself into a meat grinder. Jumping over their heads and chopping them to pieces may work in lower difficulties but on maximum difficulty you'll be dead before you can blink. Once the front line is broken, if you're out of arrows (and can't find any to pick up) grab a sword and shield or what have you and try to get behind the enemy lines and chop them up. If you have a bow and arrows, you can do the same thing from a much safer distance (remember there are often arrows to be found where you previously slew the archers on the battlements, unless they use bolts). Often times your men will break the front line and be pushed back again. In this sort of situation, you can often slip into the castle before your men are pushed back and find yourself a nice spot to pick off enemies from behind their backs (just take care not to get too close lest you be slain). You should always try to do this if you get the chance. It can make a huge difference.
At a castle defense bring a bow and three quivers and find a good spot to start picking off enemies (if it's a seige tower castle then start picking off high priority targets immediately before you find a spot). The key to castle defense is finding a good spot. Here are the characteristics of a good vantage point (I'll add screenshots later):
You want a spot that grants you a perpendicular (or nearly perpendicular) angle to your enemies.
You want a spot that gives you a good view of the upper/latter third of the ladder/siege tower.
You want your view of the lower two thirds and the rest of the field to be blocked off so that no unwanted arrows/bolts come flying into your skull.
If you try to take out targets further down the ladder they will turn towards you, lift their shields, and even return fire. Don't do this. Wait for them to approach the upper part of the ladder where they will turn their bodies to face your men at the point of entry and expose the sides of their pretty little heads to you. The next most important thing you need to understand is the priority of your targets:
1. Enemy heroes
2. Enemies shooting at you (occasionally the ballsy sharpshooter will take shots at you even from the upper part of the ladder so make sure you take him out quickly)
3. Tier 1 enemies (knights especially)
4. High tier enemies (just look at the quality of their armor to decide what tier they are)
Don't waste your arrows on low tier enemies unless its the very end of the siege. If you fail to eliminate a tier 1 knight or god forbid a hero they will rip through your forces and set you much further back. Make sure you never let a hero or tier 1 enemy to the top of the ladder (and if they do happen to get there, take them out ASAP before they do too much damage). If you can take out all the tough guys your men should be able to grind up the remaining low tier forces while sustaining minimal casualties.
In the field there are numerous good tactics you can use (and I'd love to hear some suggestions, if anyone has any) but here is how I approach a battle:
Order your men to hold their position atop some high ground.
Ride behind enemy lines while firing into them both to gain their attention and to pick off as many as possible.
Once the enemies turn to face you (and every single one of them usually turns) order your men to charge and come crashing through the enemies backs while you distract them.
Ride around the perimeter of the skirmish, pick off the enemies that are on your tail, and fire arrows into the backs of the enemies mashing together in the middle of the skirmish. If an enemy on your tail has their shield raised you can either slow your mount slightly so that they approach you and raise their weapon to strike (exposing their little heads) or simply take out their horse and leave them for your cavalry to run down. You might want to bring a decent cleaving weapon as well since as the enemy numbers dwindle it becomes increasingly difficult to score good shots with a bow. Don't waste your time trying to hard shots (mounted enemies riding perpendicular to you or single far distanced enemies) if your aim isn't true enough.

Companions
In an optimal party, two companions should be selected as INT monkeys (Jeremus and Deshavi are best suited for these roles with their base stats) while the rest are crafted into godly tanks of destruction.
Jeremus (priority: surgery, wound treatment, first aid)
Deshavi (priority: engineering [in late game], path finding, spotting)
The following image shows all companion relations.
The arrows between companions show positive relations. Companions next to each other on each circle represent negative relations.
I recommend selecting a party with no bickering in the early game so that even if your morale falls low none will leave your party. The optimal party (with no bickering) consisting of Jeremus and Deshavi is:
Jeremus, Deshavi, Ymira, Bunduk, Klethi, Nizar, Katrin, Marnid
Once you've taken your first castle your morale should no longer run the risk of dropping low enough for companions to desert you even with others that they dislike in the party. At this point you have two options:
1. Choose three more party members so that each companion only has at most one person they dislike in the party. That means choosing one member from the top circle and two from far ends of the bottom circle (you could even do this earlier in the game if you think you can keep morale from dropping too low or don't mind riding all over the place to find them again if they leave).
2. Recruit everyone. It is possible to keep every single companion in your party at once so long as you keep morale high at all times. This can be difficult but it will net you an additional 5 companions.
More info here: http://strategywiki.org/wiki/Mount%26Blade/Companions