You get one weapon....

[MetalGenocide]

Shoop da whoop of course.

Spoiler: BLAAAARG!

Oh, wait. That will take the planet out too. Hmm.
Then, a 100 metric ton of self-modifying nanobots. Then use every weapon, ever made. Why limit yourself to just one?

Cheers.

 

[BabySinclair]

The force (can be used as a weapon) which would also allow me to make a lightsaber (or two) and go to town on the hellions

 

[Daniel_Rosamilia]

AA12's of Akimbo Gloc-18's from MW2.

BOW DOWN BEFORE THE ONSLAUHT OF MY AUTOMATIC SHOTGUN!!

 

[dududf]

My BFG-9001

Oh yeaaah!

 

[RIOgreatescapist]

Ratchet's Dancing Disco Ball. DIE DANCING MOTHERFUCKAZ!

 

[EpicPanda]

I don't know about any of you, but I would want the BFG from Quake.

 

[Skooterz]

I have no need of weapons. My fists don't need ammo.

Spoiler: RELOADING!!

 

[AndyVale]

I choose Gordon's Crowbar.

I'd prefer Shepherd's Wrench.

 

[Marked Mouse]

Death Star Cannon, hand held if i'm not allowed the entire station!!

 

[NuclearPenguin]

Uhm.
The answer is obvious
A grenade launcher with a flamethrower attachment.

 

[Joshroom]

Laser minigun from Fallout 3, the one that Fawkes has. Christ that guns packs a punch; hordes of hell aint standin no chance.

 

[IankBailey]

Megas XLR because chicks dig giant robots.

 

[Raqshiem]

The Starforge.

 

[reg42]

the MIRV...

 

[Tiny116]

Orbital Strike/Death Star

 

[Hybridwolf]

The zone trooper armor, and it's rail gun. That or a smith and wesson M500

 

[bitzi61]

Gotta go with the "Golden Gun" from Goldeneye for 64!

 

[Ganthrinor]

Imperial Warmonger-class Titan.

Or Barring that, I'd take a Mass Effect 1 style AR with ME2-style Cryo Rounds.

 

[Branbeggr]

The Tool Gun from G-Mod. I'd attach balloons with 10000 in force and watch gleefully as the infernal hordes take off to heaven, helplessly dangling from thousands of cheerfully coloured balloons.

...then I'd bring out my Grinder from Red Faction: Guerrilla. Clay pidgeon shootin' with a gun that shoots exploding sawblades? Goody end times, I say.