Definitely "This is Spinal Tap" and "Best in Show".
Along the lines of "Dodgeball", I'll throw in "Old School" but only on DVD, don't waste your time watching on network TV, they cut too much out.
A couple more - "Blazing Saddles" and "Private Parts".
ETA - If you've got time to kill, get the entire first season DVD of "The Office". Not technically a movie, but still great.