I think mine's either the fantastic, original Ladykillers with Alec Guiness and Herbert Lom. I can't be bothered to watch the remake, it can't touch the original.
I'm a sucker for the 80's bratpack films too - Pretty in Pink is great as is The Breakfast Club and Some Kind of Wonderful. The John Hughes films are excellent and capture a wonderful era, particularly the music of the time.
Then again, I love the Pianist too - a gritty and incredibly profound movie with a staggering soundtrack.
There are loads of others; Soylent Green, Moulin Rouge, Spinal Tap, Strange Fruit........
I can guarantee that you'll enjoy any of the films I mention above, but if you had to see any of the films above it would probably be the Pianist.
So, if I fancy a night in what film would you recommend?