Heaven's Gate - everything about it was wrong. I honestly think Micheal Cimino deliberately set out to make the worst film ever made. Boring, impossible to follow plot, crucial dialog unintelligible due to deliberately vague sound mix, very long uninteresting scenes which do not advance plot, bizarre continuity problems, the list goes on....
Prospero's Books - unbelievably pretentious, ugly and dull. Supposedly a Shakespeare adaptation, it was basically two hours of flabby naked people lumbering pointlessly around, occasionally visited by urinating cherubs while a narrator mumbles endlessly. Intended to be director Peter Greenway's artistic masterpiece, it is totally unwatchable save for a few moments of unintentional comedy.
About Schmidt - about as much fun as waiting in line for security at Reagan International, only About Schmidt is longer and has less plot.