People can bash Leo for being a pretty boy, and anything else along that line, but, he's an outstanding actor. I look at his filmography, and I can't find a single performance I didn't enjoy. Some have obviously been better than others, but I don't believe he's turned in a bad performance. All solid films, as well... except for The Man in the Iron Mask. That stunk, but he wasn't bad.
This is a tough decision, but I'm going with The Departed. Leo's performance in this film was definitely overshadowed by what he did in Blood Diamond. I enjoyed his performance in Blood Diamond, but that character didn't contain half the depth of Billy Costigan in The Departed.
Leo stole this movie, for me at least. That may sound odd, seeing he's in nearly every scene, but he wasn't the guy getting most press heading into the release of this film. His character easily had the most emotional depth, and he did a great job conveying that. I think this is his most raw performance, the one that reminded me least of how big of a star he is.
I also dug him in This Boy's Life, The Basketball Diaries, and Marvin's Room -- three of his earlier films that most haven't seen.