Cerebral Bore in Turok on the N64. Lots of fun playing with friends, and the sound of that drill going WEEeeeerrrrerruuurrrr as it drilled into your head and killed you was priceless.
Hammer of Dawn in original Gears of War.. so great exploding giant spiders with laser beams from the frickin' sky.. man.
Hidden Blade in Assassin's Creed - so great to do a flying leap across the roofs and impale the guy on a blade that just shot out of your freaking arm -- where your severed finger no longer is..
CREEPING DARK in The Darkness - so awesome to make one of your darkness worm things slide along surfaces like a snake and then eat some guy's face..
Turrets in Team Fortress -- I loved building it and then running back and forth banging it with my wrench to upgrade it - just so I could it waste some unsuspecting scout flying around the corner..