A movie you can watch again and again... I like Pulp Fiction (aside from the painful part with Fabienne, the single most useless character in the history of movies). Kill Bill vol 1 is also a movie I'll watch whenever I happen to catch a glimpse while I'm surfing.
I'll also watch most of my favorite comedies over and over again (Animal House, Big Trouble in Little China, Young Frankenstein, Monty Python and the Holy Grail, etc.)