Brad Pitt in Inglorious Basterds,
Here is the thing, he is not that bad of an actor, just miscast. He was good in Fight Club, Seven, and hilarious in 12 Monkeys, but he sucks here cause he can't do subtelty.
Inglorious Basterds is tongue and cheek but not slapstick, its funny but also serious. The guy playing Landa can handle it, he can be joking in one moment, and deadly serious in the next. Brad can't, he just acts goofy throughout as if it was some slapstick comedy. He is unable to do that thing where an actor is making you laugh while still maintaining the seriousness of the film.