If I had to chose only one, it would be "Dirty Rotten Scoundrels" with Michael Caine and Steve Martin.

It usually only get 2 stars - so maybe you need to have a peculiar sense of humor - but I think it is just the funniest, smartest movie everl.

I laugh out loud so hard that I almost bust a gut at the scenes of Steve Martin playing the "challenged" brother. If you've never seen it and feel like a comedy tonight, I think you will love it.

I'm a big Charlie Kaufman fan, so I love "Being John Malkovich" and "Adaptation".

My favorite of all, though, is "Eternal Sunshine of the Spotless Mind", Kaufman's masterpiece. It's the most bizarre, quirky, and imaginative exploration of love and relationships I have ever seen - And yet for all the offbeat, crazy visuals and the entirely fantastical plot, the characters remain grounded enough to be entirely relatable. It's amazing.

Other good ones:
Life is Beautiful (couldn't agree more, Cassata!)
Lost in Translation
Dancer in the Dark
Children of Men
Very Long Engagement

The Graduate

Ferris Bueller's Day Off

The Godfather

Saving Private Ryan

Wedding Crashers

I have a wide variety of interests in movies, I love comedies, crime dramas/mob movies, action, drama, horror as well as the "growing up in the hood movies"

If I had to pick just one movie to say is my favorite, I would pick Ferris Bueller's Day Off. I have loved this movie since I was nine years old. It is just a funny, feel good movie that you can't help but enjoy, no matter what mood your in.

Against All Odds
Casino
Pretty Woman
Dirty Dancing
Shawshank Redemption
Any Given Sunday
The Thornbirds

One of my favorite mob flicks, directed by Peter Yates and starring Robert Mitchum-
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The Friends of Eddie Coyle

Eddie's friends are numerous, but the term friends is suspect. As a small time hood Eddie is about to go back to jail. In order to escape this fate he deals information on stolen guns to the feds. Simultaneously he is supplying arms to his bank robbing/kidnapping hoodlum chums. But who else is dealing with the feds? Who gets the blame for snitching on the bank robbers?
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The best of the mob genre - check it out.

My TOP 10
No order, I give up such stuff years ago.

Raging Bull
Blade Runner
Mo' Better Blues
Remains of the Day
The Firemen's Ball
Barton Fink
SE7EN
Doctor Strangelove
Brief Encounter (from David Lean)
If...

Pulp Fiction. There are dozens of movies I love but none have the repeat viewing qualities of Pulp Fiction. No matter what mood I'm in I would happily sit down and watch it.

I've only seen it mentioned once so far, but I'd have to go with "Dr. Strangelove or How I Learned to Stop Worrying and Love the Bomb" as my favorite movie. Easily the most genius bit of satire ever put on film.

"Gentlemen, you can't fight in here! This is the War Room!"

Glengarry Glen Ross, Pulp Fiction, Jackie Brown, the 3 GF films (I agree with Fly about 3), Casino and Cape Fear are movies I could watch over and over. I make a point of watching the 3 GF movies every Thanksgiving and Christmas.