We've already gone over the BFG.
Anyway, how about... the AK-47 from Counter-Strike!
A Galaxy Sized SMG from Halo 2 except it aint galaxy sized there, but galaxy sized SMG here is awesome, and it has a trigger size of a normal gun
Bare fists in Timesplitters =P
The beam sword in ToS, honestly what's a beam sword doing in a medieval setting?
farsight from perfect dark owns any gun ever made
the one witch has the most power would be the one that takes 100 ammo in doom 1 i forget its name coolest weapon would have to be the bfg10k from quake 3 it owns
morpho-ray from ratchet n clank man that thing is awsome
m16/mr14 in Ghost Reacon 2
The Tranquilizer from Perfect Dark!

Especially in its secondary fire.
Scarab Gun
I got it on Metropolis like 3 times so far. It wasn't easy.
I think it's the best ever too.

To you those who don't know what it is, it's a secret weapon placed in 2 different levels(Some say there is one on Great Journey, some said the person who made the vid modded.) Outskirts and Metropolis. They are hidden very well and usually require deloading/loading portions of the game to get there. They are in the form of a Plasma Rifle and when you equip it and shoot it, it makes a very huge burst flashing explosion. It's the most powerful weapon in the game. Once you shoot at a vechile or a person, the vechile blows up instantly and the person you shot at dies instantly. Just don't shoot near yourself.

How about you just say that it fires the beam from a Scarab, has infinite ammo and no overheat, and looks like a Plasma Rifle?
That weapon that destroys the universe.
How did you guys manage not to think of this?
How stupid are we to use a weapon that would take us with it?
The crossbow from Half-Life. The sniper rifle from Deus Ex. The silverballer sound supressed from Hitman 2. The shotgun from Half-Life. The DOOM shotgun. The Quake 3 shotgun. The Halo shotgun. The TS2 12-gauge. The Quake 3 railgun.
can it be a group of people or just 1 person alone? if it was a group of like 30 members then it would be the sniper rifle from halo1/2 because of the range,damage,accuracy,and how it can go through multiple enemys.
Ridiculously funny, baby shooting mobile from gunbound =).
Shouldnt this topic be dead already?
Yes, but oh well.
The Injector in TimeSplitters Future Perfect (3...2...1...SPLAT) and the shovel in Postal 2
Black Hole Cannon from Heli Attack 3.
There isn't a thing you can do to stop a Black Hole.
The Fork of Horripulation from Morrowind.
Nothing better than killing the final boss of the game with a Fork...

Redeemer from Unreal tournament series.
The Clone Commander's Chaingun from Star Wars: Battlefront II (Beta).
Or perhaps the Magma Guard's Bulldog Pistol from the same game.
Sniper Rifle/Shotgun/Vehicle Seeking Rocket Launcher (H2 Only) - Halo/2
Double Lightsaber From SW KOTOR
Grenade/Rocket Launcher - Conflict Vietnam
Arclite Shock Cannon/Yamato Gun - Starcraft
And Im Sure There's Others.
If I Think Of Any Ill Edit Them In.