Best game weapons
 

#1 - Spread Shot, Contra
#2 - Plasma Cutter from Dead Space
#3 - Volcanic Hammer from Planescape: Torment.

Yours?

In no particular order...

Painkiller, Painkiller: It's a shredder you can launch, plus it can set up a laser.
Road Sign, MadWorld: Because you're impaling people's faces with a ROAD SIGN.
God Hand, God Hand: It's defenses are impregnable. It's style is impetuous.
Nero's Arm, DMC4: Unique combat experience and the best part of DMC4.

For sheer impracticality:
The Golf Club from Persona 4
The Buster Sword for FF7
The Gunblade from FF8
The Blitzball from FF10
The cerebral borer from Turok
The BFG

For Sheer win:
The Golf Club from Persona 4
The Ripple laser from Gradius 3
The Doom chainsaw
The Laptop gun from Perfect Dark
The Red Shell from Mario Kart
The Gravity Gun from Half-Life 2
The Portal Gun from Portal

The TriRocket Launcher from Jet Force Gemini
The Excavator from Dead Rising.
The Sheep from Worms
The Mace from Twilight Princess
The Chainsaw Arm thing in Madworld is creating a lot of love too.
The Prinnies from Disgaea, dood.

Since it hasn't been mentioned yet, there is just something wicked about the Lancer from Gears of War. I don't think that's the best persay, but honourable mention.

Gravity Gun from Half Life 2, as well.

Homing+Homing from Gunstar Heroes. - It's pretty great even if it isn't very strong.
Viking Sword & Axe dual weapon from ROSE Online. - It's just plain badass, man.
The Holy Hand Grenade from Worms. - HALLEL-*ka-boom*
Meta-Knight's sword, Galaxia. - Remember Kirby and the Amazing Mirror? Where it could solve any puzzle? Yeah.

For impracticality AND win:
Fish from Phantom Brave. 'Cause nothing's more humiliating than killing your enemy by slapping them with a fish that packs more of a punch than weapons of legend.
The Steel Paddle for Siegfried/Nightmare from Soul Calibur. It's goddamn rusty and goddamn cool.
The Experimental MIRV from Fallout 3. 'Cause shooting a bunch of nukes at the same time kinda requires big open spaces.

Planet Buster from Alpha Centurai. I cheatged a bit here but the largest one wipes out AN ENTIRE SCREEN and every city/unit on it and levels terrain!
The nukes of earlier civs didn't even take out a single city.

The chaingun mortar from Tribes, dreadnought map. It fires at the rate of the chaingun but fires mortar bullets. Awesome!

The shrinkgun from Duke Nukem 3d. Never got old, never will.

It would add some depth to this thread if people explained why the weapons they're listing are so awesome.

Nevan from Devil May Cry 3: Not only is it a guitar that shoots electric bats while rocking out, it can also be used as a sword with the Swordsmaster style.

Pandora from Devil May Cry 4: It's a suitcase that can take the form of a rocket launcher, a tri-rocket launcher, a massive death laser, a machine gun, a flying death trap, and can be opened to reveal some all-powerful light a la Pulp Fiction.

Spread Needle + 99 from Phantasy Star Online; in fact PSO in general had amazingly flashy weapons,
Vigoran Flail Level 2 from Ninja Gaiden,
Spear Gun in Alien vs. Predator,
Pistols in Halo 1; the most over powered thing ever,
Morte's teeth from Planescape: Torment.

Edit: Oh, and Shockwave Blast from Future Cop: LAPD: sweet sweet shockwave!

Any automatic shotgun implemented into any game, EVAR.