Having made studies in cinema and being a long time aficionado (personal record: 7 full length features in one day), here are the ones which made the biggest impression on my fragile mind as I sat in my local theater:  

S-F Movies:

2001 A Space Odyssey

Aliens

Blade Runner

StarWars

The Fifth Element

Close Encounters of the 3d Kind

Alien

The Abyss

The Matrix

Dune

THX 1138

Twelve Monkeys

Non-S-F:

The Lord Of The Rings

Excalibur

Brazil

The Deer Hunter

Platoon

Citizen Kane

Shindler's List

Laurence Of Arabia