hmmm... as much as I would love a Mad Cat BattleMech, I do concede that it is a vehicle rather than a weapon, so the following are my desired weapons:
the Ray Gun from Nazi Zombies, such a small weapon yet able to dish out so much pain. plus it makes a satisfying 'pew pew pew'
WunderWaffal, Nazi Zombies; being able to clear out an entire room of enemies in a single shot is always handy.
PPSh-41, WWII Russian sub machine gun; don't know why, just something about the sound it makes I suppose.
and finally:
the spartan laser: an anti-tank laser with pinpoint, sniper level accuracy and range? yes please!
sorry i couldn't put any pictures, I have no idea how and would love it if some one told me how