Val Kilmer in True Romance. He got top billing while only being on screen for maybe a minute, and you never even see his face.
Winner! runner up: Lance Armstrong in Dodgeball: <object width="425" height="355"><param name="movie" value="http://www.youtube.com/v/zNJTDt9d6Kk&hl=en"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/zNJTDt9d6Kk&hl=en" type="application/x-shockwave-flash" wmode="transparent" width="425" height="355"></embed></object>
Scott Baio on Arrested Development Charlize Theron on Arrested Development Ben Stiller on Arrested Development Carl Weathers on Arrested Development Judge Reinhold on Arrested Development William Hung on Arrested Development Martin Mull on Arrested Development Heather Graham on Arrested Development (because she's freaking hot)
Probably because I was younger, but Sean Connery coming out as King Richard in Robin Hood was probably my favorite cameo, but only because it seemed so unexpected at the time.
Captain America made an appearance on the X-Men cartoon in its final seasons. Good times. Y'all don't care though.