Ben Stiller, or Jim Carrey
Generally I'd agree, but have you ever seen The Royal Tennebaums or The Truman Show? I thought those were excellent films.
And for all the people saying Avatar and The Dark Knight, I wouldn't say they are the worst films ever, you've been lucky if they're the worst you've ever seen. Both were well made, and at the very least mildly entertaining. Over-hyped and over-rated definitely, but as somebody who wasn't expecting anything from either film, I was quite happy with them.
Films that come to mind for me:
The Goods: Live Hard, Sell Hard - Saw it last night, was unbelievably terrible.
Observe and Report - Again, woeful.
There's a list as long as my arm, but those two immediately came to mind. I could probably just say "Anything with Jennifer Lopez" though, I think that's a safe bet.