If Keanu Reeves in Dracula was bad, then Coppola's Dracula started badly, and improved when his character left center stage. Except that when I watched Coppola's Dracula, it was definitely the other way round. I thought part of it was Wynona Ryder was really pretty bad in this movie. Which makes her my nominee for worst performance by a name actress in an A list picture by a reputable director (since nobody is supposed to look at anything but the director's name, we'll ignore the writers and producers.)
Women's roles are commonly underwritten, and actresses are commonly chosen solely for appearance. As a result, performances by actresses are very, very commonly underwhelming.
But if forced to nominate a reputable actor in a big budget movie with a name director, I think I'd go for Jackie Earle Haley in Watchmen. A Clint Eastwood impersonation is not even a characterization, much less a good one. If Haley hasn't got the sense and self respect to bitch about Snyder misdirecting him, he can take responsibility for it. At least, when Kurt Russell did Clint, it was tongue in cheek!
Although, since Much Ado About Nothing has been mentioned, I'm puzzled why Michael Keaton's Dogberry is so easily dismissed from the competition.