Steve Buscemi is hands down my favorite,the guy is amazing actor, and he will get huge recognition someday. Maybe with him being in the Sopranos now will help him get that recognition. I love the guys acting.
I also like David Morse (The Green Mile, Negotiator, Hearts In Atlantis) and Philip Seymour Hoffman.
Billy Crudup is also great.
EDIT: I forgot mention, Harvey Keitel, he is another one of my favorite underrated actors.