Steve's going to say Gary Oldman probably, but I think he gets a lot of respect despite his lack of Oscars
Here are a few I can think of:
1. Sam Rockwell, has done especially great with villainous roles he's been asked to play but is pretty skilled across the board. Shame he doesn't get more mainstream work.
2. Edward Norton, he's had a few clunkers (what actor hasn't?) but when he's on his game he's remarkably good.
3. Marisa Tomei. She's had a couple Oscar nominations (maybe even one win, can't remember), but nobody ever considers her an A- or even a B-lister. She's never gotten the appreciation she deserves IMO.