Trowlons, (the ones that carry you off the screen); and then Floow, (the ones that regenerate health and are made of the little rectangles. They look like ghosts). Those are both SO annoying!! I get pissed whenever I see a floow and I get even angrier if there are a whole bunch of them. The trowlons are just plain annoying, and if one catches you by surprise then your pretty much dead.
EDIT: Oops, I left out ARMANKS. I HATE THOSE THINGS.
EDIT (AGAIN): I left out the rotating skull turret things. Bokturrets, were they? Either way, they drive me friggin nuts!