The Headless Kamikaze from the Serious Sam series have got to be up there with the Medusa heads and Fleamen of the gaming world. They're obnoxious in seemingly every way possible, and there is no greater shame than when one of them manages to sneak up behind you.
These screenshots from Serious Sam 3: BFE show how they've progressed over the years. Gotta love that bodacious Serious Sam II design. The newest version looks particularly rotten, and -- oh GOD -- they have a robotic eye now.