Vote vote vote away...i'd add a poll but id only miss something out!

Alien/Aliens - Greatest Sci-Fi horror
Star Wars - Greatest Sci-Fi epic (even though i'm not a big fan)
Galaxy Quest - Greatest Sci-Fi comedy!

i reckon im the only person who hasnt seen aliens!
could never get on with star wars....although great epic i agree, that many fans cant be wrong!

Aliens - superb script, acting, FX, everything. Shame the sequels were dogshit.

The Matrix - superb script, acting, FX, everything. Shame the sequels were dogshit.

Star Trek 2 The Wrath Of Khan - superb script, acting, FX, everything. Shame the sequels were dogshit.

When it comes to ridiculously over the top bad guys Khan is king....mind you Agent Smith was pretty over dramatic by the end of the matrix.

It has to be star wars...

Blade runner directors cut

Dark city

As for the Matrix trilogy - I found the endless fights between Neo & Agent Smith rather tedious, but the rest was good.

My fave comedy scifi would have to be Escape from New York/LA or The Thing.

Blade Runner is definitley my all time favorite Sci Fi movie, though I'm quite happy to sit through any of the other Philip K. Dick adaptations, looking forward to seeing A Scanner Darkly when it finally appears on DVD.

Top 5 -

Blade Runner
Empire Strikes Back
Predator
Starship Troopers
Return of the Jedi
Wrath of Kahn
2001: A Space Odyssey
Terminator 2: Judgement Day
The Matrix
Close Encounters of the 3rd Kind
Space Balls
Back To The Future 2

(I know I picked 12, but I struggled to get it that low)

ah of course, yes terminator 2 is a very good one. good choice!

Blade Runner directors cut
Matrix 1
Invasion of the Bodysnatchers (original)
Aliens 1
Metropolis
Mars Attacks
Solaris (original)

2001 and Star Wars looked cool first time round. Now, I don't think so. BUT they changed film history

My personal faves;

Terminator trilogy, yes all of them.
Dark star; Now i bet nobody has heard of that, surreal comedy at it's best.
Silent Running; A eco message before anyone started hugging trees.
Starship Troopers; Turn off your brain and enjoy.
Spaceballs; Only Mel Brooks could take the P@@s out of george lucas and not get sued.
The origional Star Wars trilogy; Made before GL went mad, the list could go on and on but Youwould just be bored.

Gone in 60 secs
F&F
Green Mile
Philadelphia
Pulp Fiction
The Godfather 1,2,3
The Saw 1, 2
Unleashed
American History X

oh there are lots of great movies
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sry my english ....

As a couple of people already mentioned, I would vote for:

Bladerunner: the Director's Cut
Star Wars: The Empire Strikes Back (the scene on the Ice Planet of Hoth is great)
Terminator I & Terminator II
Star Trek: The Undiscovered Country
The Cube (part sci-fi, part horror)
2001 A Space Odyssey (a great film!)
Alien 1

Apart from those I can't think of many more.