Ehh, I'll probably get slated for most of these.
Titanic: absolute torture from start to finish, and that sodding song just capped it off.
Forrest Gump: I tried to watch it twice. I fell asleep half way through each time. Given my tendency for insomnia, that's a real achievement.
Anything featuring Jack Black, Will Ferrell or Owen Wilson: because they're absolutely insufferable.
Anything featuring Jim Carrey (except Eternal Sunshine Of The Spotless Mind, which was brilliant): see above, I guess.
Star: Wars, Trek, Gate, -ship Troopers... anything else with a title involving the word Star. I just don't care about space, aliens, robots, any of that nonsense.
Withnail & I: utterly boring. I got the DVD for £2 and still felt ripped off.
Anything featuring Sacha Baron Cohen: Borat is the worst film I've ever seen in my entire life. Ali G is slightly less horrendous but still awful, and I haven't seen Bruno but I didn't like the character when it was on TV so I reckon I'm justified in avoiding it.
Maybe more later, when I'm done dealing with all the hate messages and quotes telling me how wrong I am for everything on this list.