Illithids are number one for me. I fear facing them, and my players fear me using them.
Runners-up would be:
Those creepy, floating humanoids from the Monsternomicon that create mindless, pusedo-mechanical thralls. (The name escapes me now.)
Asherakes from Minions. I've used them as protagonists in a couple games. Different, interesting race that makes for an easy to hate villain race.
Kane