Hard to pick, even when it's 3
Jurassic Park - it gave us CGI, if it had been done just a few years earlier it would have had to use animatronics instead of computer effects
The Matrix (the entire trilogy) - those bullet time, time stop effects were awesome first time around
R. Emmerich movies: 2012 / Independence Day / Day After Tomorrow - very realistic ways to destroy the world
Lord of The Rings (the entire trilogy) - I'm probably going to get lynched but... I HAVEN'T SEEN ANY OF THEM, don't even care to see them
Terminator (all of them) - for a B-movie made with a very low budget with some muscle headed actor with a weird name, not too shabby
King Kong (both the original and the 2000's remake) - the original was the original horror movie, and the effects for their time were very imaginative.
Stars Wars (all of them) - the lasers, space ships, lightsabers, and Leah's hair were out of this world, literally. It set the bar for space movies
Alien (all of them) - contributed a new idea for some species of non-sentient, killing machine aliens
2001: A Space Odyssey - was Stanley Kubrik on LSD when he made this one?
Other (please post) -ah, space mutiny?
http://www.youtube.com/watch?v=RFHlJ2voJHY
At the end, I just chose, the matrix, jurassic park, and star wars