I said it in the HP thread and it's definitely worth mentioning here.
Michael Gambon as Albus Dumbledore. It's like someone told him Albus Dumbledore was an amazing, yet kooky old wizard who aspired to be the Minister of Magic, but had to settle on being headmaster at Hogwarts even though he finds kids to be rather annoying and somewhat untrustworthy.
He got better in the movies following his debut, but still...