^^ His performance in Fiza was also mind-blowing.
About Koi Mil Gaya, I didn't particularly fancy the movie much myself. But it's his performance that elevates what was a rather mediocre movie, especially his expressions. For e.g. in the scene where the bad guys hit him with the basketball and the one where he is sitting with the idol of Lord Krishna, asking what he had done wrong to be born like this. That particular scene always makes me a bit emo.
About Dhoom 2, you are right. It's because of Hrithik that the movie was successful. He has this knack of making even a mediocre movie seem good. Without him nobody would've watched the movie, even if Aishwarya would have been nude in the movie. Well, maybe a few.
I've never seen anyone give such awesome expressions. He truly is a fantastic actor.