I was hoping against odds that Joaquin Phoenix would win this, but I'm happy with Sharlto Copley. In some sense, he was given a perfect vehicle to have a great performance because his character goes through a full transformation and Copley has to struggle through a whole host of different situations. The editing in the film made for a manic story, but it also made for an over sized acting challenge because the new situations facing his character are changing how his character perceives the world. It's a fascinating performance since at the beginning his character is a vessel to the order, but the film is a thorough breakdown of that artificiality.
Often times when a film is about a character making changes, it only shows some steps of the transformation. I watched The Godfather recently again and was impressed when Al Pacino's Michael Corleone started to flirt with joining the mob by doing the restaurant killing, but there was a severe transformation only available offscreen after he came back from Italy and intigrated himself as the Don. The Michael Corleone who watches his new wife die by a car bomb and the one who approaches Kay after a couple years are distinctly different. The film wants you to assume the death hollowed out his emotions, but he repressed his feelings for over a year before going back to Kay. There is major development in that process, but yet that kind of time span difference is commonly accepted in great roles.
I give credit to District 9 for trying to fully exhibit the transformation and I reward Sharlto Copley for making every step believable. It's especially great considering he had little acting experience beforehand.