Pitt is a great actor. Interview with the Vampire. 12 Monkeys. Fight Club. Seven. The Curious Case of Benjamin Burton. Meet Joe Black. Troy. He rarely dissapoints.
Downey Jr. on the other hand is probably the most overrated actor right now, mostly because of the gazillion fangirls that his role as Tony Stark generated. I mean he became some kind of symbol of a "cool guy" in eyes of teenagers. I really like him, and he was fantastic in Tropic Thunder but he is not on the same level as Di Caprio, Norton, Pitt, Bale, Depp (in before someone rages about Depp) etc.
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My favourite scene ever by Brad Pitt: