Well, I thought about the original Star Wars, the LOTR trillogy, et al, but I simply like those because they're the best in the genre, and they possibly have the best stories...
But, for Best Movie Ever, I'm going to nominate what I personally think you should be required to watch to graduate high school...
The Big Lebowski
It's got The Dude, man! Nuff said. Oh, and nihilists. Oh, and one of the best representations of flying-dreams I've ever seen. The list goes on and on. Oh, and it's got too many subtleties to count.
As a movie, it's got everything. It's cinematic poetry; from the bowling sequence at the beginning to the dialogue, and the narrator, everything is perfect.