SF: 2001: A Space Odyssey
Horror: The Exorcist
Dramedy: Gross Point Blank
Drama: Network
Comedy, first viewing: "South Park" movie
Comedy, repeated viewings: Airplane
Fantasy: Time Bandits
Western: The Good, the Bad, and The Ugly
My all time favorite would have to be either Network or Time Bandits, because every time I see them I either find something new or marvel at the social relevance contained in either film.
Running a close second would be "Breaking Away," simply for the feel-good innocence that it portrays.