Hmm, listing three to keep it concise, I would have to say...
One Flew Over the Cuckoo's Nest
Although I appreciate the novel more in some ways, it's still an excellent adaption. Truly one of the last movies I've seen that really connects with a lot of people. Jack Nicholson's best performance in my opinion.
Seven Samurai
Although this film probably wouldn't appeal to a lot of the younger generation, I still find it to be an amazing film.
Pulp Fiction
Not much to say about this one, mostly a love/hate type of film from what I've witnessed. The way it is with Most of QT's movies I suppose.
Anyways, by only mentioning 3 I'm skipping tons of other movies, but everyone has listed lots of excellent movies. Cool thread.