Gotta be the hopped-up version of the Gravity Gun, I mean grabbing Combine from across the room and mercilessly heaving them 100 yards to bone crushing death? What could be better?
I would choose the weapon my rogue/assassin character uses in my weekly D&D game.
Rogue Blade
It is a +2 Rapier that provides the effect of the blink spell when wielded. This means that it travels through another plane of existance when striking an opponent, reappearing on the material plane once contact is made. Thus a rogue wielding this weapon gets to add their sneak attack damage upon EVERY successful hit, as long as the opponent is susceptible to sneak attacks.
Does it have to be real? If not I'd have a "weapon" that could conjure any weapon I want, as many as I'd like (I'm an indecisive person). Otherwise, so many choices: TF2 Flamethrower (for the CB), Rock-It-Launcher (for the lolz), Shishkebab (for the zomg'z), and Jarate (you never know).