The Death Star Beam is always fun, as long as you know which planet the Rebel Alliance is on. For everything else there's Alderaan, the Red Shirt Planet.
One of my personal recent favourites is the Deathclaw Gauntlet from Fallout 3. What you do is you kill a thing that's basically a t-rex fused with a werewolf and use its hand to play Nightmare on Elm Street. The teenagers are played by guys in heavy armour and super mutants.
However, as a creative optimistic psychopath, I'd say that the Portal Gun does in fact reign supreme, if only for the fact that you can hide a superweapon in the middle of nowhere and use the portal gun to launch attacks from anywhere. No orbiting the planet to reach the moon with the rebel base on it. Also, I could drop people into a pit that's nowhere near me. AND watch them fall to their death.
Once you start thinking with portals, there's no end to the possibilities!