This is a hard question! I love so many of the replies so far.
I tried to pare it down, but there are too many, so my top 5 are:
Shawshank Redemption - The most brilliant piece of work ever penned by Stephen King, turned into one of the most brilliant films ever made.
Princess Bride - I know it's kind of OTT, but I can't help it, I still love this movie so much. Nearly every line of dialog is quotable. It has a life of it's own.
Fifth element - Another brilliant piece of original fiction. I think what I love about his movie is it's stylized presentation. Every thing about it is weird, and it verges on hammy/cheesy so often it almost surprises you with it's serious moments.
Auntie Mame - My favorite classic movie of all time. Rosalind Russell is herself to the max. It's silly, it's campy, it's touching, and in the end, it has a great message.
(I Love Lucy! but not as Mame.)
And for #5, I reserve about 30 movies I'm currently trying to decide between, including Start Trek (all of them!) and LotRs ,The Green Mile, The Usual Suspects, Ever After and The Dark Crystal. How do you pick??? Argh