I've been on a pretty big Asian cinema shtick of late so I'm going to add some Eastern flair to this thread and list some very worthwhile mentions that some people wouldn't know of.
Tony Leung - Happy Together
Still his best performance yet.
Cho Seung-woo - Marathon
A huge up-and-coming Korean actor, performed as an autistic runner that's slightly based on a true story.
Ken Watanabe - Memories of Tomorrow
Watanabe in this was so brilliant, made my partner cry half a dozen times and even had me at one point reaching for the tissues.
Moon So-ri & Sol Kyung-gu - Oasis
The greatest love story that will never be told. A woman with cerebral palsy falls in love with a man who is slightly mentally disabled. Both actors went out of their skins for this film.
Song Kang-ho - Memories of Murder
I love Song to bits and he is just so brilliant in this film. There is no other actor like him on this planet.
Takeshi Kaneshiro - Fallen Angels
Kaneshiro played as a mute in this film and it was superb. Whenever he was in the scene, he commanded it every single time.
Naoto Takenaka - Shall We Dance?
Possibly the greatest Japanese actor around. His role in this film though is his highlight.
Tadanobu Asano - Shark Skin Man & Peach Hip Girl
Asano kicks arse, and he's done some excellent roles in the past but I added this one because he just rocked in every scene he was in.