Least favorite movies that everyone knows are terrible: Van Helsing and The League of Extraordinary Gentlemen (they're essentially the same movie).
Least favorite movie that many people thought was mind-blowingly awesome: Waking Life. You know when you meet someone who's a total pothead burnout and their conversation is filled with incoherent snippets of wide-ranging and entirely unrelated philosophical doctrines, and he or she tries to make some sort of important, deep, or profound point about life, the universe, Buddhism, or free market capitalism and just utterly fails to make any sense whatsoever? If that person were a movie, he or she would be Waking Life.
And I totally agree about The Blair Witch Project. Not. scary.