Been reminded of a few reading this thread: Unbreakable - terrible, Solaris - Zzzzzzzzz, Matrix Revolutions - pants and such a disapointment.
Attack of the Clones was truly terrible (much worse than The Phantom Menace), but at least that wasn't too long, unlike the first Lord of the Rings film. 3 hours is too long for any film, let alone one as boring as that.
By far the worst has to be AI. That really really sucked, and just when you thought it was nearly over there was a big insanely stupid twist at the end and another 20 minutes of tedium.