I have a few different ones. I tend to go through phases but mainly into comedy, western and mob movies.
Some of my favourites are:
The Godfather (Parts I & II) - Marlon Brando, Al Pacino and Robert De Niro... Both epic movies with 3 of the greatest actors ever. Part I lacks a bit but Part II is fantastic.
Miller's Crossing - I really like the way the Coen brothers did this movie. It had a bit of everything; comedy, suspense, twists, drama and a nice music score.
Once Upon A Time in America - Another epic gangster movie that came out of Sergio Leone turning down the opportunity to direct The Godfather to make. A really good movie that was even better than The Godfather in my eyes.
Scarface - A great Al Pacino movie. I thought it could of been better towards the middle but the beginning and end were fantastic.
Dirty Harry - Clint Eastwood in a roll he's suited for. I loved his acting and the lines he came out with. Still makes me laugh. "Well, let's have it." "Have what?" "Your report." "Well, for the last 45 minutes I've been sitting in your outer office..."
For A Few Dollars More - Another Clint Eastwood movie in which he plays a role he's suited for. I still laugh at some of his lines like when he was counting the bodies at the end to work out how much bounty money he should be making and realised he was short and thought his math was wrong till he heard the bandit come up behind him and shot him.
Unforgiven - A movie Clint directed and starred in. He made a good job out of it after other directors had shunned it.
Almost Famous - A fantastic movie following a band on tour with a journalist. Lots of laughs, good music and drama.
Back to the Future (I, II & III) - I can't help but love these movies. Plenty of action and comedy.