I assume you saw The Dark Knight? Heath Ledger gave a pretty good performance in that. He made the character seem like a real person, or at least a real comic book person, played him consistently throughout, and added some bizarre quirks to keep your interest.
I think the best acting performance I've seen is Daniel Day Lewis in There Will Be Blood. It's on Netflix if you have any desire to watch it. He doesn't have many spoken lines, but demonstrates his emotion through facial expressions. It's not over the top (anyone can show a sad face to show that your sad) but it's believable. Like you have to see through the facade that his character is putting on to know what the character is really thinking. So as an actor, he has to play a character on two different levels.
I don't know if that makes any sense, and I'm really just talking about great acting performances in oppose to great actors. But I don't really want to erase all that to go back and answer your actual question.