What's your all time favorite FPS gun?

Starting with the bests... Bows, SPAS12 shotgun, sniper rifle.

Alien Dancing Gizmo from South Park on N64. I hope that counts as a gun?

I think I ll go with this : Springfield

Railgun (Q3A) + Grenade Launcher (Q1) also Kar98k and Springfield from Call of Duty 2

Spinfusor

Death Machine from Black Ops 1

HSG Shotgun and DUAL HC from Killing Floor, and in most games MP5

Railgun, Quake 2/3 (Not 4)

BFG 9000 (doom) and BFG 10K (quake).

Browning Automatic Rifle (BAR) , Day of Defeat.
or
Gravity Gun , Half Life 2

The portal gun, by far.

Unreal Tournament's Flak Cannon. What's not to like about what's effectively a shotgun and grenade launcher rolled into one ?

The cannon from Serious Sam.

I wanted to say the Laser gun from Metal Slug but turns out that isn't an FPS.
That Laser Gun is damn good, though.

BFG 9000. Kills everything in the room with 1 shot.

The riot shotgun from Fallout: New Vegas is always a good one. Although, you can't go wrong with a good Fat Man. I don't know if I have an all-time favorite, though.

Overall? P90
Scifi: Pulse Rifle

any sniper rifle in arma 2 or 3, because sniping players at 1,5 clicks and see them trying to locate you is fun. Bonus points for sniping a heli pilot or a driver while their have passengers. Bonus points if there is a limited respawn and you pin down hostiles for 30 minutes because they cannot/refuse respawn and it takes that long for their extraction or my elimiation.

Torque Bow from Gears of War.

Edit: Err, not sure if that would fall under "gun" though.

Devastator from Duke 3d, always so much fun to use, Probably second would have to be the Drill arm from Bioshock 2, cuz my drill is the drill that's gonna pierce the heavens.

The Cat-Silenced Shotgun or Machine Gun from Postal 2

STG44 from DoD 1.3.

Portal Gun

The rocket launcher from Painkiller.
LMB = Rockets
RMB = Minigun

Not op at all.

Not a gun exactly but I absolutely loved throwing remote mines at people and detonating them midair in Goldeneye. Though in the same vein, the Phoenix in Perfect Dark was probably my favorite actual gun, even if it was just a pistol that shot mini explosions.