The iconic and legendary BFG-9000. In multiplayer, due to the ability to fire off a shot in one room, run over to another player, and then hitscan-frag them without them even knowing. They could even kill you first, and still get nailed by the hitscan! This is all possible because the BFG-9000 is probably one of the most complicated weapons in any first-person shooter.
In single player, it isn't any less powerful. Its attack covers an extremely large area and deals massive amounts of damage per hit. To normal players, they might think that the BFG only fires a big green plasma ball of death that deals massive splash damage. Actually, it's a lot more complicated. It works like this: When firing, the BFG shoots a large plasma projectile that deals direct damage (100 to 800, with an average of 450) to a single target, not splash damage. The "splash damage" comes in when the plasma ball hits a solid object and explode, in which then 40 invisible tracer rays that deals between 49-87 damage are emitted from the player's position in a cone-shaped area (which means you can hit an enemy from behind you as well) if it hits a solid object within 1024 map units. So, theoretically, the maximum amount of damage you can deal with the BFG (800 plus 49 multiply by 87) is 4,280 damage, enough to one hit a cyberdemon.
The BFG-9000 is so overpowered it turns the boss monsters into jokes, as each will die in two or three good hits (in fact the Spider Mastermind can be killed in one hit if done at point-blank range), and the final boss of the sequel was specifically designed in a way to make the BFG useless against it (provided that you're playing with mouselook turned off).
The Plasma Rifle has a very high rate of fire, great damage, and fairly abundant ammunition. Just point at an enemy, hold down the trigger, and wait for it to die. The PlayStation and Saturn ports, however, nerfed it by cutting down the firing rate to roughly half of what the PC version deals out.
The Super Shotgun of Doom 2. Incredible damage that kills even the strongest non-boss monsters in little time? Check. Wide area of action that makes it impossible to fully miss? Check. Uses the second most common ammo type in the game? Check. Is the only truly viable weapon in deathmatch? Check, for god's sake! There is a reason why more modern fan mapsets don't put it at the very second level...
Trailblazer has plenty of really overpowered weapons, but the most overpowered of them all is... the machete you start with. It starts out as a moderately effective melee slicer, but you can upgrade it to emit a Sword Beam which makes it useful beyond spitting range. Its real power, however, comes in your ability to throw it, at which point it'll gain a ridiculous headshot bonus (if playing with location-damage-enabled monster packs), the ability to fly through enemies, and a number of ricochets against walls before it instantly teleports back into your inventory. Oh, and it'll still be emitting Sword Beams as it flies around, leaving a trail of decapitated monsters as it goes. And you can easily craft three more machetes, making it possible to spam them with abandon and clear out entire rooms filled with monsters (literally, in the case of slaughter maps) in seconds.
Russian Overkill. Designed solely with the intent of making the player the most ludicrously powerful being imaginable, to the point where the most dangerous threat to them is themselves. The mod gives the player a huge array of game-breakingly powerful weapons that turn even the most daunting map packs and mod bosses into a complete pushover. Just play it for yourself, it's madness.
The Unmaker, once you find two or three secret artifacts, becomes the bane to the legion of Hell. It goes from an unspectacular laser beam to a powerful, rapid-firingspread shot that decimates enemies in no time while completely stunlocking them. Even the Final Boss can die in a few seconds from it at level-3 power with little chance to retaliate, and the aforementioned Pain Elementals with all their Lost Souls can be destroyed within a second by it without any retaliation. The scarcity of plasma ammo prevents the player from being able to rely on it as their primary weapon however, preventing it from eliminating the game's difficulty, but as long as its ammo is adequately conserved, it can get the player out of pretty much every tight combat spot in the game.
This is even more broken in the Lost Levels campaign in the Doom 64 2020 release. The final starts off with the Unmaker for later use. Unlike the original campaign, the final level also gives you the required 3 keys, sp speedrunning through the levels just to get the keys and while Invincible will make this level a much more piece of cake, even in harder difficulties.
The Super Shotgun also counts for one simple reason: it's the same massive-damage, wide-spread, common-ammo monster that it was in Doom II, except they considerably buffed its reload time, so now it fires almost as fast as the standard shotgun!
The Soul Cube is pretty damn broken, too. It homes in on the enemy with the most HP and heals you as it slices, dices and makes julienne demon slaw. Normally you don't get it until two-thirds of the way through the game... unless you're playing on Nightmare, which gives you the Soul Cube at the start (and you'll need it).
The Rich Get Richer demon rune will grant you unlimited ammunition so long as you have 100 or more armour, which is reduced to 75 or more when upgraded. Once you get good at dodging attacks, you can easily use this Rune to effectively grant yourself unlimited ammo and start spamming all your favourite high-yield weapons. This is most pronounced with the Chaingun, as it's main downside is its ammo consumption - doubly so with a fully-upgraded Mobile Turret mod, as it consumes ammo even faster, and the final upgrade prevents it from ever overheating, meaning that the lead will never stop flying as long as you don't get hit. It also combines extremely nicely with the Mastered Micro-Missile mod for the Heavy Assault Rifle, whereupon you can start creating an unending missile barrage with the power of the rune. Though, it does come at a price, with it being a particularly difficult Rune Trial.
The rune also pairs nicely with the Armoured Offensive rune, which causes demons to drop armour upon being glory-killed. Having both runes upgraded and equipped ensures that you have a means of rapidly restoring your armour to 75 if you find yourself below that threshold, so long as there are enemies around that you can quickly stagger like Possessed or Imps.
Alternatively you can just use Siphon Grenades, as long as you have full health and fully upgraded your suit's equipment capabilities, which drastically delays the waiting time between equipment uses and allows you two equipment uses at a time. Having the Equipment Power rune also assists by increasing the rate at which health is drained.
The Plasma Rifle's Stun Bomb (especially with all the upgrades) + the Super Shotgun or Rocket Launcher can get any player out of a tight jam in most situations on Normal or Hard at least. The Stun Bomb can stun any enemy (excluding bosses) and can have a player come in close with the Super Shotgun to do major damage. Or use in combination with either of the Rocket Launcher's weapon mods (Lock-On Burst and Remote Detonator) to do more damage from mid to long distance.
Unlocking all the Super Shotgun's upgrades lets you fire a full blast twice before having to reload (and also slightly increases the accuracy of the shot), allowing you to take down strong enemies like Hell Knights, Mancubi and Revenants with relative ease and making the Super Shotgun completely outclass the Combat Shotgun.
The BFG/Ammo Boost rune combo. In this game, the BFG is just as powerful as the original but balanced out by only having three shots, and it can only be reloaded with rather rare pickups around the levels. However, the upgraded Ammo Boost rune gives you a rather high chance to receive BFG ammo when you Glory Kill enemies, essentially allowing you to spam the game's most powerful weapon with impunity.
The Chainsaw allows you to one-hit kill any non-boss enemy. Once you've found enough Argent Caches to fully upgrade the Praetor Suit's ammo capacity, you can use the chainsaw to easily take out the most dangerous enemies during battles, and fuel is relatively common if you keep an eye out for it.
The Bullet Time generated by the slowdown from entering the weapon selection wheel can be exploited in conjunction with the BFG; the indirect damage rays being emitted by the big green fireball that the BFG launches do continuous damage that continues to pile up in the slow-motion moment, an effect further amplified by playing the game with graphical settings tweaked to allow as high a framerate as possible. This tactic, which is naturally exploited by speedrunners, allows you to deliberately fire the BFG so the primary big green fireball never actually connects with any enemies, allowing the continuously-emitted damage rays to basically melt through demons in gloriously smooth Bullet Time and is actually sufficient at rapidly tearing through bosses.
This was eventually patched, but there still is a bonus to bullet time, specifically in dodging projectiles. Too many coming at you for your reflexes to handle? You have full movement control in bullet time while time slows down significantly, giving you a large amount of spare time to think.
The Siege Mode mod for the Gauss Cannon. When mastered, it's basically a mini-BFG that can murder everything in the general direction in which you fire it. It's usually better to intentionally miss your target so that the beam can go further and obliterate anything that might be behind what you're aiming at, considering the generous splash damage. It quickly becomes the go-to weapon against larger enemies when the situation isn't dire enough to merit the BFG.
Pair it together with Rich Get Richer and you can dodge projectiles while charging up the superweapon in one second with about another second for cooldown, allowing a fairly high rate of fire no longer nerfed by the ammo drawback. On higher difficulties this essentially a requirement for successful crowd control, and due to none of the bosses being especially fast nor having backup can even make them easier to fight. On Ultra-Violence a mastered Siege Mode can two-shot a Baron of Hell. It can two-shot two Barons of Hell at the same time due to the blast having an area of effect wider than the beam itself, making Beam Spam a viable tactic. No weapon spread, wide range of damage, fast charge, all backed up by infinite ammo. On top of all that, if an enemy makes the mistake of getting near you when you fire, even behind you, the size of the area of effect damage on the beam will kill them, making flanking you while you're firin your lazor a terrible idea. It's basically the Quake II railgun with a power boost. Enjoy your laser shotgun with endless range.
Even the Combat Shotgun's Explosive Shot mod is exceptionally useful if you can use the arc correctly. It uses one round per shot, making it extremely cost-effective. It's powerful enough to instantly kill the lower-end enemies, who will often be clustered together. Thing is, mastering the mod turns the grenade into a cluster bomb if it directly impacts one of them. So, shooting it into a crowd can kill said entire crowd in a single round of the Combat Shotgun. Not only that, but it has a stagger effect on Hell Knights, whom are most dangerous due to their speed and power. It also does significant damage to them, allowing the Combat Shotgun to serve as a cost-effective solution for them.
Are you stuck in a particularly nasty arena which contains a Berserk power-up nearby? Equip yourself with the Seek and Destroy, Savagery and Blood Fuelled runes (preferably upgraded beforehand), collect that Berserk and begin spamming Primary Fire. Observe as you practically teleport around the arena, reducing anything that moves to torn limbs and chunks of gore in the blink of an eye.
In the beta, it's the Vortex Rifle. While it doesn't one-shot normally and actually does pitiful base damage, it can one-shot a player, if they're less than 60 health (out of 100). Which, considering its a game with health pickups and no regenerating health, can often happen after a simple gunfight.
The final game gives us the Chaingun. An automatic weapon with a good rate of fire and decent accuracy that outmatches every other automatic weapon in damage per second. Its only real weakness is its spin-up time...which is completely negated by its secondary fire to spin the barrel up at a minimal movement penalty. It's not uncommon to see half the players in any given match only using chainguns.
The Revenant Demon Rune, to a degree. Most projectiles that the Revenant shoots can one-shot most players unless they're stocked on armour and health.
The collectible Cheat Codes are this by design; they do not stop story/item progression when used and have effects ranging from obviously broken (infinite ammo, unlocking all upgrades) to silly (party mode, QuakeCon mode). The only "downside" to them is that they disable all Slayer Gate challenges when used.
The Super Shotgun returns from Doom 2016 in all its broken glory, and now has a hook that pulls you towards enemies for maximum damage. Not too bad by itself, until you factor in the Mastery, which sets enemies on fire when you hook them. Since enemies on fire drop Armor Shards, this essentially gives you a fountain of free Armor whenever you want it (a HUGE lifesaver on higher difficulties).
As it was in Doom 64, the Unmaykr is an utter monster of a weapon, capable of tearing through hordes of enemies and melting even the toughest of Super Heavies without breaking a sweat. It is reasonably locked behind completing the six Slayer Gates throughout the game and shares the BFG's ammo (which is scarce), but will likely carry you until the end of the game if you can get it.
The Sentinel Hammer, a.k.a. the Hellbreaker, in the The Ancient Gods - Part Two is quite honestly ridiculous. While it doesn't automatically kill demons in one hit like the Crucible, the tradeoffs make it arguably superior. Simply using it causes enemies to drop ammo, deals decent damage, and releases a concussive wave on impact that stuns enemies it doesn't kill. On top of that, if used on frozen/burning enemies, it dramatically increases the amount of health/armor pickups they drop. What makes it even better than the Crucible, however, is that it's rechargable. Destroying a weak point or performing a glory kill refills half a charge, a condition that the hammer itself can help fulfill by stunning nearby fodder demons. In practice, the hammer is a second Blood Punch that works even better, with roughly the same conditions for charging it. Against super-heavy enemies, it grants even more benefits: hitting a stunned Marauder with it causes it to remain stunned for far longer than normal, allowing you to wail on it with wild abandon; it can drop the Doom Hunter's shield in a single hit; and Armored Demons are rendered vulnerable much longer. Whether you simply need some breathing room, some extra supplies, or you want to end an encounter quickly, using the Hellbreaker is never a bad option. This doesn't even cover the upgrades the Hammer receives from completing escalation encounters, which increases the resources dropped by enemies and the stun period incurred by the attack.