I'll have to agree with Jim Carrey in Eternal Sunshine Of The Spotless Mind, definitely. He's always been dismissed as a comedy guy (and thus I also thought the same about him), but he really, really did a great job in here. It's the best film of his career, easily.
I didn't think of Russell Crowe until I saw this thread and the previous posts, but I have to agree. I always thought he was those kind of arrogant, cocky bastards but he did a great job in A Beautiful Mind. I cried in that show! That's how good he was.
I think Elijah Wood did a fantastic job in LoTR, too. A lot of people get too caught up in Orlando Bloom/Viggo Mortensen, and they forget Elijah. He was amazing as Frodo. Carrying the entire trilogy and portraying how the Ring slowly corrupted him was going to be hard, but he pulled it off brilliantly. It was completely natural. You felt for him. Plus, I always thought he was a low-profile guy, but this has to be his breakthrough role to date.