I'm sure that many disagree, but the worst movie that I've ever seen (parts of) is Dumb and Dumber. I went to see it with my brother, and it is the only movie that either of us has ever walked out of. We spent 30 minutes in hell trying to like it so that we hadn't wasted our money, but eventually we just couldn't take any more.
Unfortunately, because we are so cheap, we decided to walk into another movie. The only movie that hadn't been running for long was Street Fighter. Turns out, we walked out of one horribly bad movie and into another one. Awful movie, Street Fighter.
I've tried to watch parts of Dumb and Dumber when it's been on TV since then, but I can't take more than five minutes of it.
While I'm at it, I'll mention the most disturbing movie that I've ever seen. It's not poorly made or acted. In fact, the lead actress does a fantastic job. However, I had trouble sleeping for days after seeing it because it was so disturbing. It's called Lilja 4-ever. A powerful movie, but it will really get to you.