Seriously??? You CAN'T be serious!! ONE singular movie??!!!!
There's no way to chose only one........
Comedy - Clerks (And anything by Kevin Smith!), I also really enjoyed Darjeeling Limited, Royal Tenenbaums, Little Miss Sunshine, Napoleon Dynamite & I (heart) Huckabees oh yeah... and of course The Search for the Holy Grail!!!
AND Office Space!
Horror - The Exorcist (Dir. cut), 28 Days Later, The Stand, Resident Evil 1 and 3 & Something Wicked This Way Comes
Sci-fi - Dune, A.I. & Pitch Black
Fantasy - No way I can choose between all the new ones like LOTR, Harry Potter, Narnia, Eragon, etc. But, crazily enough, I think I might have to say Krull! Also Brothers Grimm and Ella Enchanted.
Anime - Nausicaa (Warriors of the Wind) or any Miyazaki film!
Drama - Hotel Rwanda (maybe this should be under horror
) also be sure to see Sometimes in April, American History X and Empire of the Sun
Romance - As Good as it Gets, Serendipity, Must Love Dogs & How to Lose a Guy in 10 Days.
Western - Dances with Wolves and Wyatt Earp
Musicals - Chicago, Moulin Rouge and the new Hairspray.
Action/Thriller - I really liked Shooter and The Professional and Seven
Forgeign - Pan's Labyrinth and Run Lola Run
Adventure - Pirates of the Caribbean movies
Martial Arts - Kung Fu Hustle
Family - Whale Rider, Mirrormask, Nightmare Before Christmas & Brother Bear. And the Shrek movies.
Documentary - Winged Migration (is that in the right catergory?) or Heart of a Lioness.
Misc.- Big Fish, Billy Eliot, Candy, Brokeback Mountain, Shaun of the Dead, Slingblade, Thumb-sucker, Trainspotting, Transformers, Willy Wonka..the old one, geez, I had better stop now...