Lots of excellent choices, but I'll be that guy...my favorite still has to be the Star Wars trilogy.
some personal favorites I don't remember seeing listed...
A Clockwork Orange
Sleeper
T2: Judgment Day
The Abyss
Back to the Future
Brazil
Predator
Starship Troopers :D
Star Trek II: The Wrath of Khan
Was I the only person who thought Solaris was overrated?
For me:
Serenity
Alien/Aliens
The Matrix (not the last two, and it would have been better if they'd just made a movie of Neuromancer)
Equilibrium
*edit* Holy crap how could I forget Starship Troopers (NOT the second one)
God yes. I love how they took Heinlein's book which was a right wing piece of shit, bordering on damn near fascism, and made a very critical satiric left-wingish movie.
Haha he would be rolling in his grave
I thought the movie adaption of Sphere was really good as well. Captured most of the aspects of the book well. Pity Crichton is an ignorant nutjob.