I think Taxi Driver is probably my all time favourite film; great story, great score, great atmosphere and great cinematography.
I like a lot of different genres of film though; I like a lot of Scorcese's other New York based films and the ones he has done with De Niro. I like Stanley Kubrick films such as A Clockwork Orange. I like 1980's American comedy in the style of the films by John Hughes and John Landis (e.g. Trading Places, Trains, Planes and Automobiles etc.) and also some of the Leslie Nielson movies of that era. I like a lot of Western films, my favourite being 'The Outlaw Josey Wales' which is an absolute classic. There are a lot of other good movies of a similar type with Clint Eastwood and John Wayne. I like some Vietnam films like Platoon and Full Metal Jacket. I like some French films (actual French films and also films for a general audience but set in France) such as Jean de Florette, Chocolat and a great French comedy movie called 'Les Ripoux'. I aslo like many other films from various genres, a few other to come to mind being Godfather Trilogy, LOTR trilogy, Run Lola Run, One over the Cuckoo's Nest, Leon, Dead Poets Society, Love on the Dole (this is a fairly old British film, I actually saw a play of it performed in Salford a couple of years ago which was fantastic but the movie is also great). If I gave it any more thought, I could probably go on forever; there are so many good films out there.