The Tyranid Hive Lord has some of the best unit synergy and excellent abilities, as well as being the game's best tank. However, what truly marks the Tyranids as a terrifyingly effective force in single player are the warriors, and genestealers to a lesser extent.

Warriors
In retribution, you generally see two classes of heavy weapons: slow-firing anti-vehicle, and suppressive anti-infantry. Blurring the line between the two is the venom cannon carried by Tyranid warriors. An immediately noticeable advantage is that you will have it from the first mission- the Tyranid warrior squad starts fully upgraded, meaning that you have more opportunities to take wargear. The venom cannon also fires at a rapid pace, meaning that missed shots have less of an adverse impact. What places it above almost all other weapons is that its rate of fire, coupled with all-around good damage, makes it equally potent against both infantry and tanks.

While not as deadly as the venom cannon, the barbed strangler offers its own benefits. As an area-of-effect suppression weapon, it can affect multiple clustered squads, allowing you to easily render horde units like guardsmen ineffective. Furhtermore, as its fire behaves much like a grenade launcher's, it's equally effective against units in cover and behind walls.

Weapons aside, the Tyranid warrior itself is a superb heavy-weapons platform. A unit can start alongside you in any mission, and with a high amount of health and three-man squads, warriors are difficult to take down and can deliver punishing fire. An immense advantage over other weapons teams is an intrinsic lack of setup time, allowing for rapid redeployment. If swarmed, they can deal decent melee damage compared to other heavy weapons specialists. The final advantage is their synapse, which increases ranged damaged for all other squads within its range. Naturally, filling your entire population cap with warriors, alongside a melee-specced hive lord and a squad of genestealers, makes a gunline that can ravage any unit it comes across.

Genestealers
While a decent melee unit even without their upgrades, genestealers begin the game without much health, and with an infiltration ability that only works when standing still. Thus, unlike warrior squads, they are best used when fully upgraded, at which point they act as an excellent ambush unit and can cripple both infantry and vehicles. The upgrades to genestealers provide the following benefits:

Adrenal rush: this ability is useful for extending the staying power of the squad, granting them health for each strike in melee and increased damage for its duration.

Rending claws: greater melee damage. Self-explanatory.

Improved Infiltration: Allows genestealers to remain infiltrated during movement, you can position them before your main attack, destroying enemy vehicles and tangling up heavy weapon squads.

One final point makes genestealers immensely flexible: the offensive track for the hive lord lets you summon genestealers for energy rather than requisition. This gives you a quick and easy way to deploy this unit into the thick of it.

Hive Lord
You get a single hero in the Tyranid campaign, but what you lack in quantity, you more than get back in quality. One immediate benefit is that the Hive lord has massive amounts of hit points, making him a viable tank from the game's very first levels, and letting him draw attention away from your comparatively squishier units. He is an incredibly useful combatant either in melee or at range, but what truly makes him broken is the variety of accessories and debilitating effects he can be given. The most recommended wargear options are as followed:

Pheromone Trail: This is the most unquestionably important wargear item. Although its stated effect of "Tyranids around the Hive Lord periodically reinforce" may initially seem underwhelming, it offers an immense advantage; you will be refunded whatever requisition/power you invested in dead troops, and reinforcement will cost nothing but population cap, allowing you to harvest piles of req/power. Pruning a few warrior squads with bioplasma at mission start will give you enough resources to take all three commander upgrades for massive health, damage, and energy increases. This is useful in any build, and should be applied at first available opportunity.

Psychic scream: An area-of-effect stun, incredibly useful for crippling incoming melee units, or weakening a force that you've just assaulted. This slows enemy infantry's movement and fire rates to a crawl, and its range is massive. Another option that will be applicable to all builds.

Regeneration: A healing-over-time ability with decent range. While not astounding, this is the only healing ability bar upgraded zoanthropes. Worth a glance for more support-oriented lords. Note that this WILL heal you monstrous creatures, making Nids the only faction whose healing targets vehicles too. If you want to run an MC-heavy force, you'll want this.

Most of the better command items for the lord act in a supportive role. Offensive synapse gives 15% to the ranged and melee damage of nearby Tyranids, as well as 20 melee skill; defensive synapse gives extra armor rating and health regeneration. Individual playstyles should decide which is best.

There is no specific armor that can be said to be absolutely game-breaking; a melee build should use Fell of the Red Terror, which gives 95 armor alongside speed and health on each kill, while ranged builds are less decisive in their options.

Talons of the Deathleaper/Shadow Scything Talons/Venomous Talons: all of these have the same primary focus, suppressing or stunning enemy infantry on hit, with the latter two being an on-hit ability and the former a chance on hit. This is effective in tying up dangerous enemy squads with low numbers, while warriors and zoanthropes kill vehicles. Any melee build should include one of these up until the final mission, where massive numbers of vehicles may necessitate crushing claws, with a fine set given by the Fortress Militant mission.

In regards to ability tracks, the best combination for standard builds is probably a straigthforward 5//5/0 in stamina, offense, and will respectively. Stamina's fourth (extra health) and fifth (attacks by hive tyrant heal nearby Tyranids and vice versa) talents greatly extend the survivability of Tyranids and give them a secondary healing source, while offense gives you the very useful charge, genestealer summoning, and multiple bioplasma shots. As detailed above with the resource exploit, talent in energy will alleviate the need to put points into will. Thus, the above build, using psychic scream, pheromone trail, regeneration, defensive/offensive synapse and talons alongside its 5/5/0 build will be able to do the following:

• Disrupt infantry: bioplasma and charge knockback, talon suppression, psychic scream. If done in sequence, this will keep most enemy units locked down for a good half-minute, if not outright killed.
  
• Cripple vehicles: Bioplasma shots into rear armor, straightforward engagement with crusking claws.
  
• Buffs: regeneration, fifth stamina talent, armor or damage to your army. Note that all of your healing abilities will work on your carnifexes, allowing you to heal vehicles at a healthy rate.
  
• Tank: Massive health pool and strong armor make the lord unparalleled in this role.

The Imperial Guard in Retribution is more or less the most broken army in the whole game. Two of their heroes are firmly in the top five, and they share a resource exploit with the Tyranids for a quick power boost in any mission, which doesn't require a piece of wargear. On the whole, very little works as well as a properly built Guard army.

Lord Comissar Bern
While not the top hero in terms of damage, Bern is unquestionably the most broken in terms of what he does with the army. Once he hits level 10, with will and stamina maxed, killing a single guardsman will make that squad and anyone in a significant radius invincible, faster, and with a decent boost to damage. This effect lasts longer than the actual recharge time of summary execution, meaning that as long as Bern has the energy and nearby guardsmen, your army is a train of unmitigatable destruction. Unlike many other heroes, this is clearly the most effective build; while there are others that can be done for fun, nothing really compares to the sheer power of that combination.

General Castor
Castor is extremely useful for his starting ability, which spends energy to reinforce a squad. As noted before, killing some memebers of a squad to regain the resources spent on them and then reinforcing them to full again allows you to acquire limitless resources and upgrade all of your commanders. Most builds for Castor are effective, as are most combinations of equipment, particularly if the trait is taken that lets his bodyguards carry the same weapons as he does, which works wonders with meltaguns and plasma guns.

Inquisitor Adrastia
Adrastia herself is not markedly useful, so I suggest exchanging her for the honor guard squad to receive an important+10 to population capacity and a squad of stormtroopers at the very start. Upgrading Adrastia to take all honor guard traits will give you a four-man squad, preferrably kitted with meltaguns and meltabombs, that suppress on hit, rapid-fire, and chew through vehicles. In addition, the extra manpower offered by Adrastia's removal will give you an extra squad to help Bern.

General Notes
The guard has some of the more potent tanks in the game, especially when fully upgraded. The Leman Russ Executioner is a prticularly fine choice for wiping almost anything off the board, with the caveat that its shots can hurt your men as well.

Given the abundance of broken heroes in your army, this army shouldn't pose too much trouble, and most any troop selection will do well.

On tabletop and in the game, chaos is noted for having THE best heroes, and retribution is no exception. Your most potent abilities and tricks will come from your heroes, who offer decent variety and unmatched offensive power among the other factions. Furthermore, you have a highly effective and cheap unit in cultists, who can grant you free units and other great benefits.

Cultists
It's important to note that cultists will never be your frontline troops, bar those moments where you're being overrun or at the campaign's very start. However, in a support role, they are peerless. Their use comes primarily from the ability to build shrines and worship at them, which grants forces in a large radius great benefits. The effects of your cultists will depend largely on the mark that you select for your lord. Most effective builds for Eliphas will use the Mark of Khorne, which also provides the greatest boost to your force. Khornate-dedicated cultists worshipping at a shrine have two primary effects. The first is a good-sized damage increase for troops in range of the shrine, which can contribute in both offensive and defensive scenarios. The second is the periodic summoning of bloodletters, and later bloodcrushers, which vanish after a period of time. The second ability is immensely useful on defense, but with the speed at which shrines are built, can easily contribute to an offensive push. Moreover, it becomes an immensely useful effect on missions with limited pop cap, such as Exterminatus and the Tyranid defense mission. If Varius's will track is upgraded to the point of letting allied infantry revive as zombies, it will apply to the bloodletters, essentially doubling their effectiveness. The other marks, Nurgle (health regen, reinforce near shrines), Tzeentch (infiltrate nearby infantry, spawn doombolts) and Chaos Undivided (First-level benefit of each upgraded track) see limited use due to being far less potent or requiring you to move shrines to the front of your force, where they are easily damaged. Generally, your chaos marines are hardy, and don't need as many defensive buffs, especially if in cover.

Eliphas
Simply put, Eliphas is the best melee combatant out of all the heroes. He does overwhelmingly more damage, heals back a decent portion of the damage he does, has a massive area-of-effect melee strike, and can mulch both infantry and vehicles. Late-game, very little is capable of stacking up to him.

There are two recognized builds for a combat effective Eliphas. Respectively, for stamina/offense/will, they are 5/5/0 and 4/5/1. The former build maxes out both Khornate and Nurglite abilities, and gives Eliphas more health-draining powers, while the latter allows him to charge into combat, reaching his target more quickly. Generally, the former is more useful, as Eliphas is relatively tough and chaos is not lacking in artifacts that give massive health boosts or ways to distract foes. In both builds however, the first thing that should be done is maxing the offense track. This vastly increases Eliphas's damage, lets him summon bloodletters from dead enemies, and upgrades his area-of-effect attack to a massive blast that kills infantry, garrisons, and vehicles with ease. After that, several points should be put into stamina for health drains and increased survivability. Weapons for Eliphas are a matter of player choice, as his damage is excellent with any gear. Accessories before the Exterminatus level should focus on lowering ability recharge times and increasing health; afterwards, one slot should be given over to the searing skull, to grant the lord a high-damage anti-horde ranged attack, while the other can be left to the player's discretion. Given his high health, the mark of Khorne is preferable as a commander item, to increase damage output further.

Neroth
While Neroth is not as much of a powerhouse as Eliphas, a certain wargear choice will make him a potent threat. The Artifact of Ahriman is a quest reward for the first Judgment of Carrion mission, and occasionally replaces Ahriman's regular shots with a burst of doombolts. Coupled with his fifth offense trait, which doubles the effects of all offensive spells, Neroth becomes extremely efficient at laying waste to horde infantry, allowing his teammates to deal with stronger foes. Outside of this ability, Neroth serves well as a support caster, using abilities to let his allies do more damage and incapacitate significant targets.

Varius
Where Eliphas is your powerhouse and Neroth is a hybrid of attack and support, Varius is best kitted out as a tank/healer. A central focus of many builds is the will track, which allows Varius to zombify all infantry he kills (trait 3), then all allied infantry (trait 4), and finally all infected infantry (trait 5). Zombies return as allied infantry, uncontrollable but nevertheless equally effective. Varius also possesses the infection ability, a damage-over-time effect on enemies and a heal-over-time on allies. Both his stamina and offense track focus on infection; the fifth stamina trait allows Varius to spread infection by proximity, while the fifth offense causes zombies to explode into clouds that heal your troops and damage enemies. Either is effective, although maximizing stamina and will makes Varius an extremely effective healer/tank. Between him, Neroth, and Kain, Varius is best suited to using a melee weapon, allowing him to distract enemy squads.

Kain
Kain is relatively straightforward, best used as an antitank specialist with a lascannon. Although not as dangerous as Eliphas, he can be made a very effective tank-buster by maxing offense and equipping armor that raises health regeneration, allowing him to have a continuous buff to the firing speed and damage of his weapon. This will leave him unable to use other abilities and continuously damage him, but results in a significant increase in damage output.

The space marines do not have as many powerful strategies. That is not to say that they constitute a weak force, but their strongest units, such as terminators, are accordingly priced, and they lack the sheer power of Chaos or the abilities of the Imperial Guard and Tyranids. However, a few wargear options and abilities do give the marines some nasty tricks.

Shadow Scale Diomedes
One of the interesting pieces of armor you will be offered over the course of your campaign is the shadow scale, a wargear reward from the capitol gardens mission. This armor offers a decent armor rating of 54, as well as permanent infiltration. Thus, an excellent candidate for it is Captain Diomedes, who, when equipped with a two-handed weapon, will easily reach enemy lines undetected and position himself with ease to eliminate vehicles and heavy weapons. A charge across the enemy line can also do ruinous damage and leave the majority of enemy infantry knocked down or dead.

Plasma Cannons
One of the best uses for the Ancient is providing him with a plasma cannon, which has artillery-level range and deals massive damage to all units in an area of effect. These advantages are further enhanced by the cannons special ability, which rains down multiple bolts of plasma from above, easily wiping out everythng in the blast area. As this is an artillery-style attack firing from above, intervening walls do not impede it. This makes it realtively easy to wipe out dangerous enemy forces without exposing yourself, even more so if you have an infiltrated spotter nearby. Certain unique opportunities, such as firing over cliffs or across chasms, will prove immensely useful on missions like Spire Golgotha and Mount Siccaris. One point ot note is that both direct and indirect fire will damage allied troops, necessitating careful target selection. Furthermore, a cannon-equipped ancient will do little to nothing in melee, so keep him in the rear lines.

Cyrus
Arguably the most broken hero in the entire Dawn of War II series, Cyrus is still incredibly effective here. Giving him a full track in energy will allow you to turn invisible and spam explosives everywhere, allowing you to solo the map with a modicum of patience, or spot for plasma cannon fire. His use is relatively straightforward: equip as many explosives as possible in accessories, grab a sniper rifle to pick off high-value opponents, and saturate the enemy in blasts while remaining invisible. Alternatively, medic packs will let you remain invisible and revive fallen commanders with ease. The only place where this tactic would conceivably fail to apply is in timed missions (exterminatus) or those where you are pressured at the very start (capitol gardens). In all, any unbreakable infiltrator is useful, and Cyrus has abilities that work perfectly with his strengths.

Note that when Cyrus is underleveled and hasn't reached at least the third will trait, it may be better to take the honor guard terminators instead of him. In early levels, enemies won't have quite as many troops capable of dealing with heavy armor, and their teleport lets them deal with enemy heavy weapons without having to close the intervening distance.