I'd say 28 Days Later followed by Sunshine, Trainspotting and 127 Hours. There was not a single aspect of Slumdog Millionaire that I found remotely interesting. It was cliched and uninteresting, I thought.
Not surprisingly, that was his most acclaimed film.
As for Sunshine, it really had the potential to be his best movie. It just got too crazy at the end. I'm all for unorthodox films and going against conventions, but it became almost like a cheesy monster movie after showing so much potential for an hour and a half.
I will never understand why he decided to take it in that direction. A voyage to the sun was interesting enough. I didn't think there was any reason to add all of that nonsense.