The classic Call of Duty Game Breaker is the PPSh-41; in the original, it had a huge 71-round magazine along with very friendly and predictable recoil and an excellent iron sight. Almost every later game tried to Nerf it somehow. For that matter, submachine guns in general have always been overpowered in the World War 2 CoD games, and the only truly noticeable nerfing any of them got was the Thompson's slight magazine reduction from 2 onward.
CoD 1 also had the scoped Mosin-Nagant; while every other scoped rifle had to reload rounds one at a time, the Mosin-Nagant used a stripper clip whether scoped or not, giving Soviet snipers a huge advantage in multiplayer.
Apart from the other broken scoped rifle, of course. The FG 42 in the original game was ridiculous; a selectable semi / full-auto high-damage weapon with an extremely accurate scope, it was a true do-everything gun with no meaningful weaknesses whatsoever.
Call of Duty 4: Modern Warfare has the grenade launcher: an attachment that can be used with any of the Assault Rifle-class weapons, is perfectly accurate, arcs over obstructions (which also handily lets it ignore the effects of cover), requires about ten seconds of practice to master, is an instant kill if you're anywhere near the impact, and you spawn with two rounds. There is no downside to using the "Noob Tube," and you essentially get not one but two free kills after every spawn.
Due to an oversight, attaching the ACOG scope to the M40A3 sniper rifle increases its base damage from 70 to 75. While this may not sound like much, the damage multipliers on shots from sniper rifles are coded so that a player with the Stopping Power perk (which multiplies bullet damage by 1.4x) will deal 98 damage on a "bad" hit, like to a target's foot, barely letting them survive a single shot with the default 100 max health. As such, the M40A3 will literally be a 1-shot kill anywhere on the body unless your target has the Juggernaut perk to increase their max health, and even then a shot anywhere that's not a limb will still be an instant kill.
Another oversight had the Skorpion's base damage unaffected by the suppressor attachment. This was bad enough in normal gameplay, where in close range it dealt 50 damage per bullet, but it was especially noticeable in Hardcore mode, where that damage is higher than the maximum health players are given, combined with the gun having practically no recoil for ranges where a single bullet won't do the trick. The weapon was universally banned from competitive-league games for this reason, even after it was patched.
The Rifle Grenades in Call of Duty: World at War. They're pretty much accurate rocket launchers. If one uses them, they will be hated.
The MP40 was laughably overpowered as well. Coupled with dual mags, and using the Stopping Power and Steady Aim perks virtually guarantees you'll slaughter the enemy.
The Model 1887 shotgun in Modern Warfare 2, prior to patching. It almost always instantly kills the target (even without Stopping Power), and can still hit at a long range despite the typical uselessness of shotguns in this series. Here's the real kicker: they can be dual-wielded. Infinity Ward themselves acknowledged the problem and issued a patch to "balance" it, drastically lowering its effective range.. Unfortunately the first patch only applied to its akimbo version, meaning a single 1887 is just as bad (or good, depending on what side of the weapon you're on) as it always was. Furthermore, using the Bling perk for two weapon attachments - Full Metal Jacket and Akimbo - would effectively give you the pre-patched version of the dual 1887s, as well as the increased penetration of FMJ. A second patch reduced this problem. A second, slightly-less-broken but still powerful shotgun is the starting SPAS-12, which due to a glitch has its maximum range fluctuate between normal shotgun range and the ranges of the submachine guns. Add Stopping Power and you've got guaranteed one-shot kills at distances other non-1887 shotguns could only dream to even register a light nicking with.
Additionally, there's the M93 Raffica machine pistol, also known as the "Pocket M16". The game treats it as a pistol, meaning it gets swapped to very rapidly and, unlike the other machine pistols, can be used in Final Stand. However, it has insane damage ratings, dealing so much damage that on its own a full burst is only not instantly fatal at the most extreme ranges - and with Stopping Power, it kills in one burst even then. Unlike the one-burst-kill assault rifles, it can also be dual-wielded to great effect, which eliminates some of the potential at extreme range in exchange for doubled ammo, letting you spray bursts even longer.
The UMP 45, unless a submachine gun is supposed to be able to kill a target in three bullets while silenced and still allow incredible mobility while running and aiming. Not to mention as well that its damage values and ranges are closer to those of the high-end assault rifles, so even at extreme ranges it'll only take one extra bullet to kill someone.
Can't leave out the Noob Tube/One Man Army/Danger Close combination. The Grenade Launcher (or "Noob Tube") is, as in Call of Duty 4, very powerful. However, you only get two grenades from spawning. The One Man Army perk, in return for replacing that class's secondary weapon allows the player to switch between classes on the fly, including to the class they are switching from, with a full replenishment of all ammo to accompany the switch. This allows an infinite stream of grenades to toss with only a short delay in switching between classes. Danger Close deals more explosive damage with a larger area of effect. Worse is that, since the grenade launcher has reliable pinpoint accuracy, it was easy to fire into the sky at specific angles from certain spots on the map to hit the enemy on the entire opposite side of the map - objective-based modes like Domination became a joke because, barring being inside a big, sturdy building, the objectives would just explode every couple seconds, making it near-impossible to actually capture them. Alternatively, if you like having a secondary weapon, the Scavenger perk allows you to refill your ammo from dead opponents, including grenades - opponents you likely just killed with your grenade launcher. It got so bad that later games never had a perk even close to resembling One Man Army, and set it so Scavenger bags didn't drop from explosive kills or replenish grenade launcher rounds.
Also notable is the G18 attached with Akimbo with the Steady Aim perk, allowing greater firing from the hip. Add Stopping Power and Scavenger and the player is given a nearly unending, highly-fatal bullet hose.
The combination of three perks: Marathon (lets you sprint infinitely), Commando (allows you to instantly lunge forward a ridiculous distance when using the knife), and Lightweight (increases movement speed). Combine with the Tactical Knife attachment (which lets you carry your knife alongside your pistol to significantly shorten the delay after knifing someone), and there will always be someone running at inhuman speed and stabbing people from across the room for one-hit kills every time. The worst part is these three perks are unlocked by default.
As overpowered as it, it isn't as broken as it used to be. Holding a Care Package marker (or a Sentry Gun/Emergency Airdrop marker, but everyone used Care Package because it's only four kills) would grant a speed increase that would stack with Lightweight. Effectively, you'd run at roughly 200% speed with the ability to slide ten to fifteen feet towards someone for an instant kill with a knife; lunging also grants invincibility frames if your opponent doesn't shoot fast enough. To top it off, as you're running so damn fast, on everyone else's screen you'd be crouching while running so you're even harder to kill.
Add the above-mentioned UMP .45 to the 'knife class', and you get a class that can not only sprint indefinitely at higher than normal speeds and lunge across rooms for instant knife kills, but that is also a credible threat at range.
The chopper gunner can also be seen as quite overpowered as it lets you easily spawn trap opponents, leaving them easy prey in most maps (some maps with high cover, such as Skidrow, Karachi and Invasion, prevent this).
AC-130 can do this, too, but typically doesn't have equal killing potential because it has to reload. The trade-off is that the AC-130 is significantly harder to shoot down than a Chopper Gunner. Not that the guys on the ground would notice. Unless some guy has a class with MANPADS, nothing is going to cut the rampage short save the gunner getting bored.
Prior to patching, the Javelin missile launcher had this reputation. Before the patch, the Javelin could be used to get revenge kills using a glitch causing the Javelin to launch into the ground immediately upon death, killing anyone within 20 yards.
Some of the more controversial game-breakers in the series are found in Call of Duty: Black Ops. The Famas and its expy, the AUG, are both hated and loved for their insane rate of fire (second only to the G11 within its bursts), powerful damage (second only to the Stoner 63 as far as full-auto DPS is concerned), and high accuracy. The AK-74u with the Rapid Fire attachment also gets a lot of approval and disapproval. Although it gets some extra recoil (countered by the also common Grip attachment), its rate of fire is increased to a rate that competes with the Famas and AUG, essentially making it a lighter yet slightly weaker version of them. What makes the Famas, AUG, and AK-74u so controversial is the fact that they're unlocked rather early (14, 17, and 26), so higher level players generally don't have the upper hand just because they're at a higher level, which was considered to be one of the problems with the aforementioned Akimbo Model 1887s.
A fun experiment is to go into a game lobby and to examine the statistics of all of the players there, and to count the number of people who the two guns they've been killed the most with are the Famas and AK-74u. Hint: It'll probably be much easier to count the people who haven't been killed most by those two guns. The AUG usually competes for the third spot with the Galil, which doesn't have quite as fast a rate of fire as it but competes with less recoil and a slightly higher mag capacity.
Modern Warfare 3 raised the bar with SMGs; particularly the PP90M-1 and MP7. The former had incredibly fast fire rates and could hold a massive 54 rounds with Extended Mags, becoming a proverbial bullet hose (note its nickname was the 'Super Soaker' or 'Water Pistol gun'; the latter had virtually no recoil, and the recoil it did have was easily mitigated. It was common to see players being sniped by a no-recoil silenced MP7.
Another class of weapon that was game-breaking was the Machine Pistols. MW2 had overpowered Secondaries in the Shotguns, but MW3 pushed the bar up a few notches with their pistols. Being able to have what amounted to free Overkill meant that players very rarely took any other secondaries, save for the occasional anti-aircraft launcher. FMG-9s were the worst of the four in terms of balance. Having the same fire rate, range and capacity as the aforementioned PP90-M1 was bad enough, the only saving grace being its insane kick; but the Game-Breaker was when it was used akimbo alongside Steady Aim. Having two submachine gun-class weapons hipfired meant players could essentially use them as a primary weapon out to a middle distance, the output more than making up for the range drop. It was so bad, the gun was nerfed multiple times over the course of the game's lifespan, but even despite these it's still pretty much just as bad as it was at release. All of the other machine pistols are insanely good as well, with fire rates, recoil, and damage values equaling the best primary SMGs, with the only other one getting any sort of nerf being the returning G18, with greater, more wild recoil and a reduced capacity from MW2 - as mentioned, it's essentially the Overkill perk from CoD4 for free.
Two weapons were broken by design: the Type-95 and the Mk 14; the former had the highest automatic damage in-game, albeit in a 3-round burst, but the bursts occurred so fast that it was a non-issue, and it was also incredibly accurate. The latter had players take advantage of the lack of firecap to use a modded 'turbo' controller to have full-auto versions that were incredibly accurate and powerful.
Lightweight, a.k.a. "Crackhead Pro", where players moved so fast that they could dart around the corner and kill you before you even saw them. Because of the power of this perk, 3 perks became the go-to class: Lightweight (faster movement and no damage from falls), Toughness (flinch much less when shot) and Dexterity (faster movement over obstacles, recovery from melee, and aiming from a sprint). This combo was so powerful, it became virtually impossible to play well without them, and worst of all, like the game-breaking perks of MW2 they were all the default perks.
On the weapons front, the Target Finder became this for many players. It highlighted anyone without Cold-Blooded with a targeting diamond, and was more used than the Thermal Scope because it allowed for an immense amount of peripheral vision, no filter making it difficult to discern the background, and quicker aiming time, especially with the new Quickdraw attachment. This was most used in tandem with the below two.
The LMGs received a massive buff in this game. Not only were the majority of them incredibly accurate, but the ability to use 3 attachments at once allowed for the ever hated Grip/Target Finder/Suppressor LMG combination.
Call of Duty: Ghosts had integrated attachments, meaning certain guns had traits that were automatic, meaning you could potentially have 4 attachments on a single gun. Of note was the integrated suppressor, which gave certain weapons unmatched playability, as they were always silenced, and could run 3 attachments over the standard 2, gaining an edge over non-integrated weapons. The Honey Badger was the most prevalent, being the go-to weapon for most players when starting a new character, given it also unlocked the majority of stealth perks for no cost, making a very powerful class available from the get-go.
Assault Rifles gained a massive buff over earlier series counterparts due in part to the game's more mobile gameplay. Thus, having six guns with minimal recoil and consistent damage output makes them the go-to weapons in Mulitplayer.
Snipers were broken for a whole other reason: due to the incredible amount of lag in the opening few weeks of the game's lifespan, two of the guns became so overused to the point entire lobbies of players were using them. The main contenders being the MORS and Atlas 20mm, the former for being a single shot with great handling, the latter for being a One-Hit Kill almost anywhere at the expense of no hipfire. Except the MORS had an insanely quick ADS time, and the Atlas could break its no-hipfire coding via quickscoping.
Supply Drops give both cosmetic rewards and weapon variants. The latter is a major factor, as many of the strongest gun variants, which are the source of the above gamebreaking weapons, are classed in the Elite category. This turns the Supply Drops into a Luck-Based Mission to find one of these Elite guns to even stand a chance of competing.
Prior to its nerf, the EM1 Laser Rifle was also this, thanks to its insane accuracy and rate of fire plus unlimited ammo. The only thing keeping it down is the overheating after firing for some time, but there's an upgrade for that. Adding salt to the wound is the weapon has a fast rate of fire based on the frames per second of the game (Meaning the weapon will fire faster or slower depending on your frame rate) and getting a buff in a patch. Thankfully, its fire rate was lowered.
The War Machine, if not already OP in multiplayer, is even more broken in Campaign. Unlocked at Level 15 (about 70% through the Campaign, if you are playing on Hardened or above, earlier if you take a run or two through the Combat Immersion training simulation), it comes as a secondary weapon, with 8 shots in the cylinder and 8 in reserve. Unlike the previous iteration of the War Machine in Black Ops 2, Black Ops 3's War Machine shoots a grenade which, upon hitting a surface, releases three small balls in a large spread that do a small hop and then cause a massive explosion. With the War Machine, all you have to do is find cover, shoot above the cover at varying angles, and in no time everyone on the other side of that cover is dead. What makes it broken, however is its semi-automatic fire rate, and doubled with the fact that it does 3x the damage if your target is in close range. Top it all off with the fact that most boss enemies (like the P.A.W.W.S) can be taken out in four shots, which roughly amounts to about 1.5 seconds. And to end it off, if you have the Copycat tac-rig equipped, have fun getting infinite ammo every 2 seconds.
The BlackCell rocket launcher in multiplayer. It comes with four rockets, can lock onto targets (all Scorestreaks and the Safeguard robot) quickly, and due to its quick reload, is able to be spammed repeatedly at targets. Many players have found it frustrating that their high-tier Scorestreaks are destroyed as soon as they enter the battlefield especially if the BlackCell user is equipped with the Engineer perk, which lets them see enemy scorestreaks on the mini-map. The BlackCell is also capable of direct-impact killing enemy players caught between the launcher user and ground targets (ground-deployed Scorestreaks and the Safeguard robot). As a result, the launcher's reload speed was nerfed while scorestreak health was increased.
The "Holy Trinity" of submachine guns in the game, aka the Kuda, VMP and Vesper. Each has their own quirks, but each absolutely dominates the field in their own special way, even against assault rifles. The Kuda is an all around submachine gun that is good at range and in close quarters, with good accuracy and damage. The VMP has a beefy 40 round magazine to start, a high fire rate and excellent damage, meaning it will win in most toe to toe engagements. The Vesper has a 30 round magazine like the Kuda, but has an obscene fire rate, making it unmatched in close quarters. Add in the Fast Mag or Extended Mag attachments, and its nigh unbeatable. It's not unusual to see games dominated by someone using one of these three guns, usually with a 2.00 or more kill/death ratio. As a result, all of the submachine guns were stealth nerfed in a patch with the Vesper getting the absolute worst of it. This video sums up the dire situation the submachine gun class is now facing.
The P-06 burst sniper rifle has extremely high penetration power, making it a force to be reckoned with if FMJ is attached. On a more general note, snipers have ZERO penalty from suppressors in this game, in contrast to the range/damage penalty of other games, so you could get shot through a wall and not even know where the shooter came from!
While the Specialist weapons in general are intentional game breakers in the right hands, some are far and away stronger than the others.
The Reaper's Scythe, a minigun that folds out of his arm. Not only is it insanely accurate and extremely lethal, but this thing can utterly wreck scorestreaks. Even a poor player can easily get quad kills without much trouble when using it and a good player can wreck an entire team twice over. The only balancing factor is that it is the slowest charging special in the game.
Firebreak's Purifier isso insanely overpowered that it's ridiculous. Unlike the Scythe where getting a kill requires a few seconds of aiming, a player using the Purifier only has to graze a player with the flame and it's an automatic kill. It's not unheard of Purifier users to set their sensitivity to insane levels, run into a room filled with enemies, spin around really fast and come out with 7 or 8 kills. Facing a Purifier user at mid-range or closer is absolute suicide, as most of the time the player cannot do enough damage in the time before the flame kills them. Oftentimes, even if you kill them first, the flames will graze you enough for them to pull a Taking You with Me.
Battery's War Machine, as above, is your average Noob Tube taken Up to Eleven, being easy to aim, spam, and able to decimate entire rooms with its grenades, whether it's a direct impact or from a split grenade. Flak Jacket only protects against the latter.
Ruin's Gravity Spikes comes out fast and has a deceptively large range. It's slightly mitigated by the fact that it can be interrupted by killing the player in midair, as well as the fact that many players telegraph when they're going to use it by boosting as high as they can before they land, in an effort to make themselves harder to hit but leaving them open to attack longer.
Spectre's Ripper is basically Modern Warfare 2's Commando lunge all over again. It also has an insanely fast recharge timer.
Prophet's Tempest. You will learn to fear its distinctive zapping sound. Anyone hit with its beam will be instantly killed before you can say "FUCK!" And if there are many players grouped together, that's bad news, since the beam can spread and chain targets.
A later update added a 10th specialist to the game, Blackjack. When he gets a kill, he can obtain the scorestreak of the person he killed, and swap between different ones. If that's not bad enough, it only takes 5 kills, which means you would have Purifiers dominating a whole entire team on the second round of a game of Search and Destroy. And if the person who is playing Blackjack is a very good player, he could get a Game-Breaking weapon every 45 seconds. Compare that to other Specialists, which can take up to a couple minutes minimum to acquire their abilities. He is balanced by only having 60 minutes of in-game time to play as him, and the fact that you can only unlock him by using "Blackjack Tokens", which are obtained by doing very difficult weekly challenges. Still, a whole team of Blackjacks can absolutely break the meta.