I love Uma Thurman, and think she is an excellent actress.
Ben Affleck has done some horrible stuff, and at other times he has done quite well, I wouldnt place him as one of the worst actors although he certainly not one of the best. I dont consider Miriah Carey or Kelly Clarkson either to be a real actress, so I am going to try and go mainstream with my choices.
Vin Diesel acts how he is supposed to act, It may be bad, but honestly, no one would like him if he was a good actor. So here are my choices
Paul Walker - he was mentioned earlier and I agree completely
Keanu Reaves - seriously, his best acting job was in Bill and Ted's excellent adventure
Kevin Costner - although he did partially redeam himself with Open Range. Waterworld and the Postman will haunt him forever.
Halle Berry - I agree, notioriously bad over actor, and the only reason she got an Oscar was because they were trying to give them to all the black people that year.
Steven Seagal - It has to be said, but honestly, I like his movies.
Jennifer Lopez - she just stinks.
Thats my list, Im sure I could think of more.