Philip Seymour Hoffman for Capote. I love that guy but Heath Ledger deserved it for Brokeback.
I think retrospectively it stings a little. I do think Ledgers was the better performance, but of course less showy. However at the time I think people just assumed his time would come....
As I said above, at the time i was fuming about Crash winning best picture over Brokeback, the one and only time I've been genuinely angry about the way an awards ceremony went.