For me?
Either The Last Airbender or Dragon Ball Z The Movie.
How 'bout you?
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I would say The Room, but that one is so obnoxiously bad it's impossible not to enjoy it. My least favorite that Ive personally seen is, by far, Eragon. They took a book I enjoyed and they chewed it up, spit it out, put it in a shredder, incinerated it, swallowed it, and then vomited it back up into a script.