I think Johnny Depp perfectly executes any character he is given - particularly one by Tim Burton, and it goes without saying that Jack Sparrow was a phenomenal performance, oh, excuse me, Captain Jack.
I think when discussing the best actors in the movie, everyone is quick to overlook Emma Watson and Daniel Radcliffe, but to be honest their acting really grows toward the fourth movie. Especially given how young they were and how un-relateable Harry's character must have been, I have always been impressed. But I also really love Maggie Smith. She was so cute in Divine Secrets of the Yaya Sisterhood!