Gary Cole. Didn't even know his name for years, but one of my all-time favourite actors.
It kind of upsets me when actors with comedic chops don't play to that strength and vice-versa. George Clooney is freaking hilarious, and if you don't believe me you haven't seen O Brother, Where Art Thou? Robin Williams is not, but was brilliant as a creepy villain in Insomnia and One Hour Photo.