For me, sometimes, it's a close fight between Pulp Fiction and Fight Club. But I say Pulp Fiction took the trophy for me.
I don't have just one but these are some of the best movies I've seen
Second Hand Lions
Talledega Nights (yes I know its not near the level of the other 3 but I'm a car guy and I love the movie)
I've no favorite movie of all time but I can list some of my favorite movies.
The Odyssey with Armand Assante. [I just found out that it was first introcuded as a TV series but the first I saw it it was presented as a movie]
Batman: Under The Red Hood
I also like The Goonies