Some people can't count to 5.... LOL
So, there are LOTS of great, groundbreaking, classic Sci-Fi films out there. Many of them won't get mentioned simply because you have to pic the 5 best.
Here are the 5 BEST (life altering dreams):
Star Wars (original 3, technically if you spliced them, they'd be 1 big long movie!)
Star Trek (all of em)
Terminator (all of em)
The Matrix (all of em)
The Road Warrior (all of em)
Honorable Mentions:
The THING (cutting edge in its day AND John Carpenter ("the original Steven King"))
Avatar
A.I. (Artificial Intelligence)
E.T. (VERY important film when you are 9 yrs old)
Jurassic Park
Back to The Future
Hey, I didn't say I could count to 5 either... LOL
IGN has a pretty cool listing:
http://www.ign.com/articles/2010/09/...es-of-all-time