The most universally agreed-upon Game Breakers are Maggie (a revolver with a shotgun spread like the Mashers from the first game, most desired with the "Two-Fer" prefix that makes it shoot twice as many shots at the cost of 2 ammo), The Bee (an Amp shield that does not drain the shield when you shoot Amp shots), and The Sham (an Absorb shield with at least an 80% chance to absorb bullets, adding them to your backpack and doing no damage to you).
With patch 1.2.0, The Bee got a heavy nerf. No longer does it have the insane amp damage, the recharge delay is longer, and the recharge rate is lower. In addition for all amp shields, on multi-projectile weapons such as shotguns or the aforementioned Maggie, the amp damage divided by the projectile count then added to each projectile. That being said, SMG's like Slagga or Lascaux still have the bee bonus for every single projectile. This did completely remove the "so hilariously overpowered it's bad" Conference Call shotgun + the Bee combo. (Conference call generates additional projectiles on hit with an enemy which ricochet back onto where you hit with it. The first raid boss died in seconds to this combo)
The Sand Hawk SMG from Pirate's Booty is now the weapon that gets paired with the Bee. It fires 8 projectiles (in the shape of a bird that actually flaps its wings as it flies through the air) that each get the full amp bonus, and it's a Dahl weapon so it burst fires.
More specifically, The Bee (and all amp shields) have their damage divided by the number of projectiles listed on the item card. This means that unique weapons that have interesting ammo-splitting properties not listed on the card will retain the full amp damage for all of projectiles. Sand Hawk, Unkempt Harold, and one variant of The Fibber can get enormous damage boosts from splitting projectiles.
A combo that can obliterate anything other than raid bosses with 2 seconds of firing (that includes lvl 52 super badasses) is Infinity (high fire rate pistol with infinite ammo and no need to reload), the aforementioned Bee shield, and the Sherif's Badge (a unique relic which increases pistol damage by ~20% and firing speed by ~55%). Even better if you have a class mod that increases firing speed and/or pistol/gun damage, alongside skills with damage boosting effects. Zer0's 0ne Sh0t 0ne Kill skill stands out, since it gives +60% damage to every first bullet in a clip, which applies to every single shot of Infinity. With the right set-up this can lead to doing over 100k damage per shot.
And another weapon that becomes a monster with such a set-up is a specific variant of the Fibber, namely the version whose bullets split into 9 on ricochet on the first non-enemy they hit. Each projectile gets the Bee damage added, despite the "fix" which prevents additional projectiles to get it.
The Buffalo. Like the Elephant Gun from the first game, it's a sniper rifle that does an ungodly amount of damage but has no scope. Unlike the Elephant Gun, aiming down the sights causes the camera to zoom in enough that you can still use it for long-range shots.
Anarchy + Close Enough = massive amounts of damage. What makes this truly ridiculous is that while Anarchy and Close Enough are on seemingly opposed skill trees, they're also both Tier 1 skills; you can have Anarchy with a maxed-out Close Enough long before level 20. Your accuracy still sucks though, but even then one could easily rampage through hordes of enemies if done right. Gaige even lampshades it when you get enough Anarchy stacks.
Torgue weapons count for both grenade and gun damage boosts. Axton has many skills that boost both. Axton + Torgue weapons = so many explosions.
Gaige's Blood-Soaked Shields talent gives her a substantial shield refill (maxing out at 100%) in return for a hit to health (maxing out at -5%) every time she kills an enemy. It's intended to turn her into a type of Glass Cannon, and it works pretty well for that purpose. However, combine it with Captain Blade's Otto Idol, which restores a percentage of health every time you kill an enemy (at the cost of a shorter Fight For Your Life time), and the health hit is completely obliterated.
Evolution, an elemental resistance shield with a very, very high resistance and health regeneration. Most shields in this category resist anywhere from 10-30% of elemental damage; Evolution usually has around 75% resistance. It's only downside is a lower-than-average capacity, low enough that a level 50 Evolution will not be practical for carrying into Ultimate Vault Hunter mode.
Krieg is a very difficult character to play, but properly specced, he is ridiculously overpowered. The top-tier skill of the Mania tree is Release the Beast, which changes his action skill so that when activated at low health, you instantly recover all health and become a mutant badass psycho with fifty percent damage reduction. When it wears off, the action skill automatically refreshes—meaning you can do it again immediately if your health falls again. On top of that, speccing into the Bloodlust tree grants the Taste of Blood skill (damage reduction of up to fifty percent while using the action skill), and Blood Trance (duration increases up to twenty five seconds), meaning that for upwards of half a minute you can turn into a berserking psycho nearly immune to all damage who regains health on every kill...and then turn right around and do it again. It's very telling that Krieg players are the only people who seem to avoid dying every other second in Ultimate Vault Hunter Mode.
It is very possible to run through certain "dungeon" levels using absolutely nothing to your name. No guns, class mods, grenade mods, shields or relics. Just your assigned skills, your melee button and a whole lot of violence.
The Vladof Sniper Rifle Droog comes with it's manufacturer's characteristic rotating double-barrel giving it ridiculous firing rate as well as paired with a high count magazine, effectively making them assault rifles with ridiculously high base damage, as well as highly accurate when scoped. This is fair enough for most classes, since the limits on the amount of sniper rifle ammunition are very strict compared to most other types of weapons, meaning that even with full ammo upgrades a player will usually only be able to use a couple of clips before their ammo is depleted. The problem arises when these sniper rifles end up in the hands of Salvador, who rapidly generates ammo during his ability and can possibly pick up Ammo Regen as a perk for his class mods; and because no-scoping is still fairly effective at close to medium range, he can use it in place of an assault rifle or SMG. By itself, this ensures that he will effortlessly cut down almost anything in his path, since his ammo regen is almost perfectly on par with what he's burning through; combined with almost any skill in the Rampage tree, especially "Inconceivable" or "5 Shots or 6", which drastically decrease the odds that he'll even need to reload for the duration of his ability, meaning he can fire without pause for as long as it lasts, turning him into a Instant Death Radius on legs. And as if that isn't enough, he'll still have a second gun on hand during the rampage, meaning that he could possibly use TWO of these at once if he's lucky enough to find them. Worse still is the "Lyuda/White Death" Legendary Variant of the Droog, which not only take the damage and magazine way past 11, (Up to over 70000 damage on UVHM even without the bonus from all Vladof parts, and 42 rounds with the proper class mod, skills, and Badass ranks) but makes the bullets split into three after a certain distance.
The Antagonist is basically an Infinity Plus One Shield. Of course, the only way to get one is to farm raid bosses on Ultimate Vault Hunter Mode until you have enough Seraph Crystals to buy one, meaning you likely won't have anything to use it on except for farming the raid bosses some more.
There's an exploit that lets you kill the storyline final boss, The Warrior, with ease. You start on a bluff with The Warrior in a crater below, which you need to leap down to start the fight. If you die, you're sent back to the bluff near an ammo vending machine, and The Warrior continues his attack pattern. To prevent you being able to shoot The Warrior from the bluff after the battle starts, a large shield pops up that blocks all shots. However, not only is the shield not tall enough to keep some shots from hitting the very top of The Warrior, but the cliffs surrounding the shield have cracks in them, letting you easily take potshots at The Warrior when he leaps to either side. Thus, you can just stay on the bluff and shoot The Warrior until he dies, with no chance of any of its attacks hitting you. You don't even have to worry about running out of ammo, since you start next to an ammo vending machine!
There is also an exploit where it will make the Fibber (A Hyperion pistol with sarcastic stats) deal the amount of damage that is says on the card. Meaning that a level 20 fibber could potentially deal the same damage as an ordinary level 45 Hyperion pistol.
Pre-Nerf, the Bee and the Conference Call combo. The Bee is a shield that boosts your damage provided you have full shields. The Conference Call is a shotgun that spawns additional pellets on hit or after traveling a certain distance. The two of them together, especially with Salvador, allow people to solo the Bonus Boss hands down.
The Bee shield has since been nerfed to be less effective with multi-projectile weapons like shotguns, but from the first DLC comes a replacement weapon to pair it with: the Sand Hawk SMG. Although it fires eight shots at once, the weapon stats don't say so, meaning all eight projectiles benefit from the full damage boost.
Love explosions? The Commando class with a Torgue weapon and perks geared toward grenade and assault rifle damage can find a weapon that in the inventory stats does 600, and in practice does upwards of 2000 damage with chest shots.
Gaige the Mechromancer's Close Enough, full stop. Her Ordered Chaos tree normally makes her the worst character in the series in some situations, but by spending 5 points in her "Close Enough" perk on the opposite skill tree, Best Friends Forever, 50% of Gaige's bullets usually hit the enemy anyway for reduced damage. But since Gaige hits harder than Brick on steroids, whose punches can cause the Unreal Engine itself to quake, and is very compatible with Tediore SMGs as an Anarchist...And this is from a skill intended to help new players make strategies and help them learn Borderlands 2's accuracy. Not to mention, going further than this and focusing on Ordered Chaos while spending extra points in Best Friends Forever may result in a Mechromancer who replenishes shields from killing people, regenerates health outside of combat, can put Axton's "Rain of Lead" style of gameplay to shame in sheer mass of bullets, makes each bullet hit like a freight train, and half of them ricochet and hit people anyway. Gaige could theoretically gib people from a Tediore pistol.
And with the Ultimate Vault Hunter patch, it gets worse. Not only can she now ricochet bullets that hit enemies at other enemies without suffering penalties, but taking a build like this with a Legendary Class Mod or a Slayer of Terramorphous Class Mod will result in carnage the likes of which has never been seen before. With the right class mod and skills like Potent as a Pony, Myelin, and Discord, Gaige becomes not just a machine of pure devastation at 300+ stacks, but she also becomes a tank.
The Mechromancer's special ability Deathtrap is also insanely powerful thanks to the Best Friends Forever skill tree. Among the powers it grants are: 20% Cooler, cooldown reduction, allowing a player to summon Deathtrap more often. Upshot Robot, an ability that extends the length of time that Deathtrap can be in effect, based on how many enemies you and it can kill, and this ability boosts Deathtrap's strength with each and every kill. Explosive Clap, a special ability that can wipe out entire groups of enemies instantly. Buck Up, The ability to recharge a character's shields in seconds. Oh, and Deathtrap can levitate and move through walls, and can shoot lightning bolts at flying enemies. The final skill in Best Friends Forever, Sharing Is Caring, will give Deathtrap a copy of your shield, which, depending on the shield you have, can be a devastating power to have. Your Robot Buddy can be death incarnate for enemy hordes.
With the Ultimate Vault Hunter patch and the level cap increase, it gets crazier: each still tree has a few perks that boost Deathtrap's skills in some way. At high enough levels, Deathtrap gains increases to base melee damage, movement speed, laser beams, chained melee attacks, the list goes on and on.
Infinite ammunition weaponry. Extremely rare and, if discovered early on, you'll swiftly outgrow their base damage, but once you find them, there's not much that's going to stop you for the next twenty or so levels.
Krieg with grenades and explosive weapons. Strip The Flesh boosts the damage that all explosive attacks can do. Blood Bath gives a bonus to weapon damage based on the number of bloodlust stacks you have if you kill an enemy with a grenade... or an explosion. The increase is .5% per bloodlust stack, meaning that after a kill, you can get a boost of up to 50% weapon damage. Salt The Wound increases shotgun damage up to 20% based on damage taken when shields are down. Add all of that together with a class mod that boosts explosion damage and/or the skills in question, and with a Torgue shotgun, Krieg becomes a murder machine, dealing double the damage that his weapon would normally deal. Even without Salt The Wound, a Torgue Pistol or Assault Rifle is a weapon of mass carnage in his hands.
Alternately, Release The Beast, the final skill on the Mania tree: Trigger Buzz Axe Rampage when at 33% health or lower, and Krieg instantly gets full health, 50% damage reduction, a huge increase in melee damage, and as soon as the rampage ends, Krieg can instantly trigger another rampage, normal, or superpowered if you were knocked down to low health while you were taking out enemies.. So if you were reduced to 33% again, you can trigger another transformation. You can quite literally chain this over and over until every enemy in a wave is dead. 33%, by the way, is what you'll normally be at when you come out of Fight For Your Life mode.
Krieg's Bloodsplosion skill, combined with a Roid shield and a Rapier, is ridiculously powerful as well. Bloodsplosion blows up enemies after you kill them, adds overkill damage on top of that (which is why you want to maximize your single-hit damage in melee), and does 5% more damage per bloodlust stack (maximum 100, for a 500% boost in damage). The increased explosive damage and grenade damage from several of Krieg's skills, as well as those from relics, similarly apply to the explosion's damage. What makes this game breaking? Bloodsplosions chain, and the overkill damage stacks. One high damage melee attack on a weak enemy can clear a room. Add slag into the mix, and you're doubling not only the damage from the melee attack, but the damage from the Bloodsplosion. Used correctly, with properly-grouped enemies, Bloodsplosion can take out several of the game's raid bosses in 2 attacks (one slag, one melee), as the damage keeps increasing as enemies explode, eventually reaching the display cap of 999999k.
In Mister Torgue's Campaign of Carnage, there is a set of racing side quests, the last of which is lvl 49, and is almost unbeatable... almost. In another sidequest, you can get a relic that improves your vehicle's boost, so you can boost longer and more often. It is possible to win the side quest with this relic equipped, and the exp reward is quite substantial, given that the quest is so high level. The quest is infinitely repeatable, and takes 2 minutes, win or lose, with no enemies to fight save for the much lower level standard fodder that dwell in the Badass Crater Of Badassitude. If you master the course, you can level up extremely quickly.
In Tiny Tina's Assault on Dragon Keep, there is a gun called the Grog Nozzle that does about a fifth of the damage of comparable weapons, but you will regenerate health equal to 65% of any damage you deal (from any source) while wielding it. That on its own is pretty good, especially if you've got a good action skill or grenade. However, on top of that, there's a chance when you pull it out that you'll get "drunk," which drops your accuracy and causes you to fire six bullets instead of one, for the cost of one bullet. Even better, if you switch weapons, the effect remains for a little while, so you can multiply the damage of any weapon by six. Combine with Gaige's Anarchy and maybe a Bee shield, and everything dies very quickly. The only downside is that it's a mission weapon, so if you turn in the sidequest, you lose the gun. However, since the actual reward from that chain of quests is lackluster, most players just never turn it in.
Grog Nozzles dropped legitimately from the Gold Golem for one day during the $100,000 Loot Hunt, meaning not only can you finally turn the quest in, but you can actually get one that has the prefix you want and have it stay that way (the one you get during the mission is randomized when you load up the game).
The Norfleet and Sham combo. The Norfleet is a legendary rocket launcher that fires three powerful balls of energy with a large Area of Effect. The Sham is a shield that has a chance of absorbing incoming gunshots or explosions into your ammo pool all the way up to 94%. The AoE from the Norfleet's energy balls are large enough that, at the proper range (close but not too close) and with the Sham equipped, it is possible to reabsorb your own rockets as you fire them, effectively giving you infinite ammo for perhaps the most powerful rocket launcher in the game.
Many people also combine the Sham with a low-levelled Logan's Gun, basically trading pistol ammo for rocket ammo and not instantly downing themselves on the rare occasions the Sham doesn't absorb the ammo.
Gunzerking + Rubi + Infinity = god mode. Specifically: all of Moxxi's signature weapons heal their wielder by a percentage of damage done to an enemy while they're the active weapon; Rubi has this effect to the greatest degree short of the Grog Nozzle. The Infinity is a rapid-fire pistol which doesn't consume bullets and never needs reloading. Combine the two, and as long as Salvador has something to shoot at, he can get near-constant health regeneration that can keep up with any enemy short of a raid boss. If he's got a slag variant of Rubi, it goes Up to Eleven because the slag effect will increase the amount of damage done by the Infinity, which will increase the amount of health Salvador leeches from his target, turning his shields into an afterthought.
Many will opt for a Double Penetrating Unkempt Harold over the Infinity. If Rubi + Infinity makes Salvador a god, then Rubi + DPUH makes him every god in every religion put together.
The Bonus Package (and its Seraph Counterpart Meteor Shower) is a hilariously powerful grenade mod that spawns 2 times the amount of child grenades on explosion. This, in of itself, is usually good enough to carpet bomb an area and leave nothing but smoking corpses. However, the mod is capable of being spawned with the "Homing" and "Longbow" prefixes, sometimes even on the same mod. Moreover, if you're gunzerking and have the Double Your Fun perk, you can chuck two of these at once. All of these combined means that chucking the grenade even once will ensure that every enemy within the immediate vicinity is reduced to a fine red paste.
Any type of multi-barreled shotgun will most likely stay in your loadout for a long time, especially 4-barreled guns. They fire multiple round, which are further made up of multiple pellets. Combine that with the high damage-per-pellet of shotguns in general, and you won't need much else. The only downside is that it eats up a lot of ammo - one round for each barrel per shot - but that can easily be overcome with SDU upgrades and certain skills. A Jakob's Handy Quad, for example, found at level 5 will still be extremely useful at level 20.