Here're my personal top 10, mostly from the 1940s and 50s (some later)
Casablanca (possibly the best of all time)
Citizen Kane (a close 2nd and Orson Welles at his best)
Sunset Boulevard (the best Hollywood movie about Hollywood ever made)
High Noon (for me, the finest Western ever made)
Saving Private Ryan (finest WW II film)
The Maltese Falcon (another Bogart flick and possibly the best private eye flick of all time)
Chinatown (nearly perfect script; Jack Nicholson in top form; almost rivals Maltese Falcon)
Shindler's List (awesome)
A Man for All Seasons (how to make Tudor England seem not only fascinating but actually relevant)
An American in Paris (best musical)
Singin' in the Rain (best musical)
Ok, ok, so that's 11 not 10. I can't decide which one is my favorite musical.