Conker Live and Reloaded, does this with the Hogster, The Grunt's upgrade weapon: It's essentially a grenade launcher witch can fire in three different "modes" being Frag (standard grenade properties, bounces a bit and then 'splodes) Impact (explodes as soon as it hits the ground at the cost of reduced range) and Release (press the fire button after shooting to detonate whenever for increased range), the real gambreaker is the Hoogster does insane amount of damage regardless where you are when it explodes, Its blast radius and Splash Damage are so damaging and so far, if a grenade is even close to you expect to die (especially if you're a lower health class). Now if the Hogster only had the frag mode, this wouldn't be so bad as it takes quite a bit of time to explode, so the faster weaker classes have ample time to get away, but its Impact and Release mode make it so where one can detonate them as soon as it lands (in Impact) or while still in the air (for Release), combined with its large damage, you can literally kill anyone you can see on screen. The gun has almost no drawbacks, seeing as how ammo is unlimited in this game, you only have to worry about reloading. While it does have a long reload time (but still not the longest in the game) and a 2-shot clip, you can pretty much kill the entire team with both shots (or at least anyone posing an immediate threat).
This has pretty much lead to hosts either banning upgrades (the item that grants a Grunt a Hogster) or banning the Grunt class and leaving upgrades on (for the other less game-breaking class).
In Saints Row, at least the first one, the .44 Sheperd is a gamebreaker. Six-shot magazine, and 250 bullets in all. There is a reason why this is so overpowered, and you can collect ammo from the most common gun, pistols. And the .44 Sheperd is available very cheaply and quite early in the game. And it looks cool.
In the 2nd one, the .44 Sheperd is a 2 hit kill, but you can carry one in each hand, with the same ammo capacity ad the other one. That's still 127 times One-Hit Kill if you fire both at once. There really wasn't any reason to use anything but the .44 in either game.
You can also unlock infinite pistol ammo by doing a certain sidequest. Your magazines aren't bottomless, but your ammo pool is infinite.
Speaking of the infinite ammo unlocks, getting infinite ammo for assault rifles also gets you infinite ammo for a 25mm sniper rifle that kills anybody in one shot anywhere. And, if you do stronghold missions as soon as they're available, said sniper rifle is given to you for free relatively early.
Star Wars Battlefront 2: The Wookiees. They had above average health, access to drones, a grenade launcher. The Wookiee crossbow is a combination of a sniper rifle and a shotgun! The ability to do heavy damage up close and get a spread with this weapon and with charge it increases its power and range. Access to grenade launcher allows a Wookiee to go against vehicles adding to the insane versatility of the berserker. The only class barring hero that counters a Wookiee is the shock trooper who's shock gun can do more damage but lower health, reload and charge times means you have to have a really good escape.
Heroes as well count, particularly the ability force push. Ordinary characters cannot block force powers and even with another hero blocking still doesn't work against force push.
The only thing worse than Force Push is Force Pull. i.e.: "I bring you into lightsaber range and leave you helpless for a few seconds at the same time."
Saber throw is a mite excessive in the overpoweredness department, too. Especially considering that it was only used twice in the movies with a total kill count of one.
Most of the award weapons. Award guided rocket launcher allows you to stand behind a cliff out of any enemy's line of fire and pound them with a barrage of high-explosive missiles. Award rifle fires three-shot bursts, two hits (or one headshot) will take down anything. Award shotgun has about twice the effective kill range of a normal shotgun. Award pistol does as much damage as a normal sniper rifle, while you carry 96 shots and can make 16 shots before reloading. Award sniper rifle lets you kill an enemy by hitting them in the toe (assuming the game decides to count your shot as a hit).
The original Gears of War had a rather overpowered shotgun that most people used exclusively, regardless of their range or the weapon their opponent was holding (which was usually a shotgun as well, so it worked out). Naturally, the community decreed that the shotgun was the only weapon that took any skill, and decried every other weapon in the game as a Game Breaker or "noob weapon". They Changed It, Now It Sucks took off in full stride when the shotgun was nerfed in the sequel (to the point where a large number of players abandoned the game for being too "noob-friendly" and returned to the original as they couldn't handle this "dying" thing that was happening to them whenever they tried shotgun rushing through Lancer fire or firing back at it from medium-long range). Those who stuck around for the sequel learned to abuse the Hammerburst to its fullest potential until the Lancer was buffed to be able to compete with it.
Except then they just started to make sure they played Execution mode exclusively, so that while someone with a Lancer could down a shotgunner at a distance, he'd still have to run in to melee range in order to make the kill, invariably bringing him into shotgun range of the rest of the opposing team (who are all using shotguns of course). Planting frags as proximity mines also became completely ridiculous very early and even smoke grenades could be used as such to knock you down long enough you'd be shotgunned to gibs before you could get back up.
The smoke grenade was later patched and does not have knockback. The shotgun, however, is still the most abused weapon in the game. Many players adopted the "Two-piece" strategy in which you melee the opposing player, then blind fire a shell into their legs directly after. The combo results in almost guaranteed instant kills if all rounds from the shell hit. which at point blank where this tactic is supposed to be done, isn't very hard.
The shotgun was so broken it got two weapons to counter it; the Sawed-Off and Retro Lancer. The former is better at close range, and requires less skill to aim, but it's even worse at any real range. The latter has one of only two instant Executions in the game, meaning players using it don't have to hang around and expose themselves.
S4 League's Counter Sword is arguably a Game Breaker; with several clicks of the mouse, you can unleash a combo of melee hits that knock your opponent back, and it also has an oversized fist that you can use as a shield. It's no wonder that detractors dub it the CS Spam.
The Semi-Rifle is an automatic rifle with a zoom feature, giving you the range of a Rail Gun or Cannonade but without the need to aim as carefully. It doesn't even recoil.
Forget about the CS. The Twin Blades are almost impossible to beat in the melee only mode when wielded by a moderately decent player.
I-Fields in Mobile Suit Gundam Climax UC follow the anime and completely nullifies all beam weapon damage. Funnels also follow you around and peck at your hp. By pecking I mean blowing huge chunks out of you.
BIONICLE Heroes is completely and utterly broken from level 1. Masks count as "lives", and if you get enough LEGO pieces, you go into "Hero mode" where you get infinite health, and only goes away at scripted areas in the game. All weapons have unlimited ammo, there are at least two respawning masks (read: lives) around wherever there are enemies (including bosses), all enemies do maximum damage of 1/2 a heart out of five IF (big if) they hit (also includes bosses), hearts refill whenever an enemy dies or whenever you switch masks (which you can do instantaneously), and you can get Hero mode without leaving the first room of the first level. Convinced?
And then at the end of the game; you get the Vezon character, who has the most powerful weapon in the whole game.
A good case could be made for the Support in Monday Night Combat. He has a One-Hit Kill ability (airstrikes + Shotgun on Assault and weaker), a mobile turret that is ridiculously powerful, and of course there is bunny-hopping to take advantage of laggy hit detection and using a weapon that requires literally no aiming (heal/hurt gun).
The Sniper could also be considered extremely overpowered in the right hands. His explosive bullets are great for crowd control and can take out entire mobs of robots in only a few headshots, his flak grenades took out turrets quickly and can hurt enemies behind cover, his ice mines make getting headshots on enemy pros incredible easy as long as you could bait them into the mines, he is able to grapple jackbots just like the assassin, and Gunners and tanks were easy to instantly kill due to their giant heads. His Grapple was insanely overpowered, killing many pros in one hit and knocking others who survived off the stage. While his grapple power was eventually Nerfed, his knockback was not and could still easily kill enemies up close. One would think that the melee grappler assassin would be able to easily kill a sniper, but with gold armor only a grapple from behind could instantly kill him which wouldn't happen in most cases. This allowed the sniper the chance to grapple back and instantly kill them. The only thing that could truly challenge a good sniper was a good Assault.
The relatively obscureSuper Buster Bros has the gun powerup. It fires at such a ridiculous rate that it basically results in everything on the field being wiped out in 5 seconds. It only has two downsides: the fact that the game only allows one powerup at a time, making the powerups suddenly more of a threat than the enemies, and the fact that it can't destroy terrain (a pointless weakness, as most maps don't require it and those that do will often drop other powerups).
Earth Dependent on the difficulty level. Some weapons that you unlock will tilt things in your favor, but only until you start the next difficulty level. Pretty much the only TRUE Game Breakers are the final weapons unlocked on Inferno.
The Force Gun of Dead Space 2 is extremely ammo-efficient, especially once given damage upgrades. Even on Zealot, a player using nothing but the Force Gun and possibly one long-ranged weapon will find themselves with a constant resource surplus and able to effortlessly clear all opposition in the game.
In the first game, many players were content to use the very first weapon - the Plasma Cutter - exclusively, to the point where people would go "Stop Having Fun" Guy on people who used other weapons, calling them morons who were playing the game wrong since the Cutter outshined every other weapon by far.
Red Faction has the rail gun, a high-power sniper rifle which can shoot through walls. Even without ammo the gun's scope still works, allowing the user to see enemies in different rooms.
Transformers: War for Cybertron has the Leader Class, who gets some of the best weapons in the game and an ability that both increases their damage and armor called Warcry. Basically a juggernaut that can kill you in two shots. The basic strategy is to blow Warcry and bunny hop while spamming either the Fusion Cannon or the Magma Frag Launcher. They were eventually nerfed slightly, but they're still pretty broken.
All Points Bulletin has the N-TEC 5, an AK-lookalike that dominates the entire game at all ranges. You've heard of spray-and pray? With the N-TEC, you don't need to pray, because you know they're going down unless they have an N-TEC as well.
In the Reloaded version, the pre-nerf Colby PMG submachinegun held the Gamebreaker status for months. High accuracy while moving? Check. High damage? Check. Being able to outgun every other SMG, shotgun, and even rifles at their optimal range? Double check. It was not uncommon to see pre-made teams fully equipped with this weapon.
The OSMAW, a shoulder-mounted rocket launcher, also has the dubious distinction of being too effective. The launcher itself only carried two rockets, but it would destroy almost every car in one shot. The cars that could survive a rocket were often so low on health that any friendlies could shoot it with their normal weapons and destroy it in seconds anyways. It generally requires a near-direct impact to kill players, except that most firefights take place on streets, with cars being the only available cover. Also, cars produce bigger booms than the rocket itself. A single OSMAW rocket can take out an entire team given the right conditions (ie. enough cars lying around). And considering the need for driving cars to get to objectives, the OSMAW is rarely in situations where it's not effective.
In BloodRayne, the first game's implementation of the Blood Rage mechanic makes Rayne virtually unstoppable. Time slows down, Rayne speeds up, and she inflicts so much damage in such a short amount of time that almost any non-boss enemy will be instantly killed or maimed, and most bosses will have their health drained very quickly. The Bloodlust gauge is also refilled by any melee attack, making it easy to replenish.
In the old Catacomb Fantasy Trilogy, the X-Terminator is meant as an area attack hitting enemies on all sides, but because it just uses a cloud of normal projectiles going in all directions, it can be used to make any boss or other fight trivial by going next to the opponent and launching it a few times, letting dozens of projectiles hit the single opponent.