Twilight - I've never seen such wooden acting since Captain Scarlett (and yes, he was a wooden puppet).
Final Destination - Okay, the first one was good. It was a new concept and I enjoyed it but after that it got old fast.
Saw - Again, the first one was good but please, stop.
Harry Potter - I enjoyed the books but the films skipped so much it took away from the feel of it. To top it off they took the worst book and made it into a 2 parter. Seriously, come on now, they spent 3 quarters of the book just teleporting around and the last had a bit of a fight. How the hell they managed to make 2 films worth out of it I don't know.
Any remakes - Come on now hollywood, the films you are remaking didn't need it nor do a lot of us want it. You are ruining classic films.