Least favorite book? Twilight saga. Never read the books, I hate them on principle. If you're gonna jump on the vampire bandwagon, do it right, dammit. If you're gonna make something that is emo, gay (or at least bisexual), and that sparkles in the sunlight, call it something else. For more reasons to hate the book, see any of the various Twilight drinking games. This one's my personal favorite, spoilered just because.
The Great Gatsby takes an honorable mention for worst book. I actually enjoyed the book until my English teacher ruined it with symbolism. White dress? Symbolism. Nice car? symbolism. Clock? symbolism. Ivy? Symbolism. Tuxedo? Symbolism. Party? Symbolism. Billboard? Symbolism. etc. etc. ad infinum. I mean, come on people, sometimes a clock is just a fricken clock, y'know?
worst game? I'd have to go with just about any MMO. I don't mind dealing with people, but when you give them the anonymity of the internet and the ability to do mean angry things to you, I generally tend to avoid it. There are a few gems out there that are excluded from this for me, but not many.
And as for worst movies.... Twilight movies for the same reason as the books.