Duh, Citizen Kane!
Vertigo would be in my top ten. How about Singin' in the Rain? Or Raging Bull? Godfather? Casablanca?
There are lots of movies that I love which wouldn't make it into my top ten. Top ten favourite films is just too hard!
What about Bonnie & Clyde, Synecdoche New York, Pan's Labyrinth, No Country for Old Men, Juno, The Deer Hunter, Fargo, E.T., Amadeus, Apocalypse Now, Mississippi Burning, Who Framed Roger Rabbit, Crash
, Do the Right Thing, Raider of the Lost Ark, Ran, Tokyo Story, Platoon, Malcolm X, Schindler's List, Being John Malkovich, JFK, Almost Famous, Monster, Nashville, Chinatown, Woodstock, 2001: a Space Odyssey, Ulysses, etc, etc?
Then there are the lesser known ones, like: Black Stallion, Hoop Dreams, My Dinner with Andre, The Right Stuff, After Hours, House of Games, Three Colors Trilogy and Z?