What springs to mind immediately is:
Darknuts, Leevers, and Keese (LOZ series)
Cliff Racers (Morrowind)
Teras Kasi Clone troopers (the dicks in the Jedi Temple in the SW3 game)
The shotgun/body armor pricks from Black
Lambent Dretches (GoW)
Hydralisks (Starcraft)
Murlocs (WoW, WC, love/hate with those guys though)
Any ghost in the Fatal Frame series
Marlboro (Final Fantasy)
Boomers (L4D...they always barf on you at just the wrong time)
Gungans (Star Wars Galactic Battlegrounds...I always seem to have trouble with those pricks)
Tiger Woods (20xx PGA tour series)
Most of the enemies in The Force Unleashed that are actually worth noting at all. Seems in that game the only challenge comes from some random BS ability or defense that interrupts the flow of choppyslicethrow.