There are so many good movies it's hard to say just one. A movie I really like that I wouldn't mind seeing again is Training Day. A recent movie I've seen that was great was Role Models, it was hilarious.
Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb
Best movie I've ever seen, has an absolutely amazing cast including Peter Sellers, George C. Scott, Slim Pickens, and a VERY young James Earl Jones. Its a great movie with a lot of satire on the Cold War and the arms race in general and is tied for first for greatest ending to a movie ever for me (the other being Clockwork Orange).
Musical vs Perverted 41 year old trying to dance with a 16 year old...
Grease wins...
Comedy films for me
Waterboy
Happy Gilmore
Spongebob
Hot Fuzz
Final Fantasy VII Advent Children
Scream 1,2,3
Scary movie 3 (Obviously the best one...)
Jeepers Creepers
Most comedy/scary films
I am a "movie fan" so its hard to list just one....
But If I Must: Fight Club
Some others in the list:
Terminator 2
The Matrix Trilogy
Star Wars Trilogy (The ORIGINAL trilogy (not that re-edited crap with added cartoon characters))
Every Kevin Smith Movie Ever (Especially: Mallrats, Dogma, & Jay and Silent Bob Strike Back)
The Stand (book > movie, but movie still good)
Die Hard 1, 3, & 4
The One