I've only ever walked out of the cinema once and that was watching The Exorcist; wow was that film boring!!
Pacific Rim - It's so bad I've had to re-watch it several times just to see if it's as bad as I thought;
The New Godzilla, mind the old one isn't any better
Dungeons and Dragons
Aliens 3 & 4
Predator 2 & Predators