Scooby Doo is one of my least favourite EVER. Batman and Robin was almost as painful, though had some camp humour to it. Bruno was just stupid and unfunny. Plan 9 from Outer Space, Santa Claus Conquers the Martians, and Surf Nazis Must Die were terrible movies, but also wonderfully hilarious.
The most disappointing movie that I've ever seen is Crash. I was hoping that it would be a complex meditation on racism in modern America, but instead it was a jumbled mess of different overacted cartoony characters learning oversimplified lessons about racism. UGH!
The movie I've probably hated the most in recent memory though was Extremely Loud and Incredibly Close (aka Extremely Lame and Incredibly Corny). That movie was obviously trying to use the subject of 9/11 to pull at the heartstrings and earn awards, but just came off as melodramatic, cheesy, and self-important. The main kid in it was particularly annoying throughout the movie. I thought it was a travesty that the movie was nominated for Best Picture alongside the likes of such great films as The Help, Midnight in Paris, and Moneyball. It seemed like it was nominated solely because of its subject matter, rather than any actual merit. The only good thing I can say about the movie is that Christopher Plummer was excellent, as always.