Artificial Intelligence. Why? This dumb movie kept me wanting to watch it more and more hoping it would get better when it only got worse. Most bad movies I can quit 20 minutes into it, but I watched the whole thing and wasted more than 2 hours of my life (it was long too).
An honorable mention for the worst movie award goes to Wedding Crashers. My aunt that watches everything didn't even like it. I myself slept through it.