One person mentions Casablanca? ONE PERSON? Ye gods, what is this world coming to?
In any case, my picks would have to be (the aforementioned film excepted):
Fear And Loathing In Las Vegas
Breakfast at Tiffany's (another all time classic that I'm surprised nobody's mentioned. What's the deal, guys? Just too embarrassed?)
When Harry met Sally (also a tad embarrassing, what with me being a large, bearded, heterosexual male, but what c'n I say? It never fails to get me, and it's up there with Casablanca for great endings)
The Big Lebowski
The Usual Suspects
Fiddler on the Roof
Camelot (Richard Harris version)
About a Boy (I know, a Hugh Grant movie. But in spite of all Notting Hill-related prejudices, it's a nice story, and the man's a damn good character actor)
Bugger, that's... what? Eleven? Twelve, counting Casablanca? I'd better just stop there. But to all who read this: if there's any of these movies you haven't seen, see them. You're selling yourself short if you don't.
Four Colditz salads and a Hermann Goering.