I haven't really seen any movies in theaters that were extremely bad, but if I had to pick one that I wasn't crazy about, it would be Cars. I was excited going into the theaters to see it, but I wasn't very happy with it in the end.
I watch it now, and I cringe heavily at the multitude of piss and poopy jokes. No movie even comes close to this shitfest. I can't believe I at one point even enjoyed it.