I'm with Pete. It's to hard to name just one. Here's my list:
Amadeus, Fight Club, When Harry met Sally, Groundhog Day, Scarface, The King of New York(Chris Walken rules), The Natural(Best Sports movie ever), Meet Joe Black(Great acting and story and it has Claire Forlani(Wow, in it), The Terminator(especially the first half). Sleepless in Seatle(Love Meg Ryan). Gladiator.There's a lot more I'm leaving out. If I had to name one I would probably say Amadeus(Greatest genius to ever live) But there are so many great ones.