If you could have any video game weapon...

Disagree. Even things that are bolted down can be weapons with the Portal gun. You could put a blue portal on the outside of a plane and then drop orange portals under people you don't like while it's at 30000 feet. Or maybe have someone fall through their room infinitely (portal the ceiling and the floor) then put the ceiling portal on one of the walls in your room so they splatter (and probably also break) one of their other walls.

With the portal gun, every medium-sized (and some large, if you're creative) object as well as momentum itself are your weapons. Also I think if two people were given these weapons, the Portal gun guy would win.

Try that when you have a radiator flying at your face.