I have to go with Keanu as well. For his absolute worst performance (and to disprove Haggan's comment that 'he can [only] act what people write for him') see him in 'Much Ado About Nothing'.
Excellent adaptation of the Shakespeare play, and his lack of ability is highlighted by being up against Kenneth Branagh, Emma Thompson, Richard Briers and Denzel Washington (amongst others).
My least favourite female actress is Julia Roberts, although I have to say she redeemed herself a bit with 'Erin Brokovich'. Prior to that, I'd only seen her laugh, cry and scream her way through every single role she played.
I also have to disagree with a couple of folk who suggested Jim Carrey. While he did earn his reputation as a 'kooky' actor, check out his performances in 'Man on The Moon', 'Bruce Almighty' and 'Eternal Sunshine of The Spotless Mind'. Real depth and versatility, and I do think that 'The Truman Show' was grossly underrated due to his reputation going before him. Just my opinion, as ever!
Edited, Thu Sep 23 18:11:11 2004 by Kelanthor