Star Wars. I saw it well over 150 times - that's just IN theaters. This movie above ALL others can easily be said to have altered my life simply because it gave form to what it was I didn't know I wanted to see in a movie.
Close Encounters of the Third Kind. I saw this in the theater a LOT as well though I never kept count. It was in a theater across the street from where Star Wars was showing. My brother and I would leave school, drive downtown and see Star Wars, then walk across the street and see CE3K. At least a couple times a week for several months.
Bridge on the River Kwai. I saw this on TV on the Sunday Night Movie when I was very young. I remembered very little of it and understood even less what was actually happening, but the imagery of the sky filled with bats, the sounds of the jungle and the river, and the magnificent closing scene with the Colonel falling on the detonator and destroying the bridge and train were indelibly imprinted on me. I saw it again on TV several times but didn't fully appreciate it until I was much older.
Bite the Bullet. The movie in general is not all that stellar, but Gene Hackmans performance is fantastic and the part itself is a exceptionally intriguing character. I particularly love his monologue to Candice Bergens character about "charging" up San Juan hill (really about his Cuban wife), and his reactions to Jan Micheal Vincent's characters abuse of animals.
A Bridge Too Far. I bought this DVD for no particularly good reason than I was accumulating a lot of WWII movies at the time. It's overly long and not well paced at times but I love it.
Aliens. The Single Most Quotable And Quoted Movie - Evar. The Godfather sucks in comparison. I can watch this one ANYtime.
Deep Impact. Far and away the best Asteroid Disaster movie you could name. Fascinating because it has little to do with the nature of the disaster itself but with how the characters react to impending disaster and doom.
Into the Night. People seem to have utterly overlooked this neo noir comedy with Jeff Goldblum and Michelle Pfieffer. I think it's great.
To Live and Die in LA. I first saw William Peterson in Manhunter and think it was at least equal to the sequel Silence of the Lambs. This movie, however, was a real action movie and Peterson probably did his best work ever in it.
Real Genius. Hilarious, a little thinly plotted at points, but otherwise shockingly well-written and maybe even insightful as far as the problems of science untempered by philosphy.
Shawshank Redemption. As a believing Christian I'm naturally a sucker for stories of redemption, so the addition of this being an otherwise wonderful movie even if you just take it as a prison-break movie, character study, etc. makes it quite possibly my favorite movie of all time (despite having seen Star Wars so many times.)