Many of the other movies in this thread are fantastic in their own right and objectively better. Interstellar, Moon, Contact, all great films to me and I own (or will own, for Interstellar) all of them. But when I first saw Avatar I was completely blown away from the effects and the cinematography. It was the first "modern" 3D movie I'd ever seen. The story line was simple but I tend to get wrapped up in the plot when I'm watching movies and for Avatar I was nearly stunned walking out of the theater based on the experience as a whole.
I know the plot is very derivative. I know that it's morals are ridiculously blunt. But dammit I never felt that way after watching a movie before or after, which is why it remains my overall favorite.
As a personal close second choice and one that I don't think anyone will mention; The Wrath of Khan. You have to be a Star Trek fan of some sort to truly appreciate the characters, but it is the epitome of Star Trek on the big screen and just a great pseudo-revenge story.