[Ttvetjanu]

I know restricting it to 5 is hard, but give it a try. Honorable mentions are of course allowed.
 
In no particular order:
 
Dark City
The Matrix
Star Wars (the old trilogy)
Gattaca
Twelve Monkies
 
Honorable mentions: Aliens, Total Recall, Terminator 2, Blade Runner

 

[MuppetFace]

Not to sound like a film snob, but the original La Jetee was better than 12 Monkeys by a considerable margin. A lot shorter too. You should check it out some time.

 

[oqvist]

Serenity but it requires that you have seen the FireFly serie before or it doesn´t make sense .
 
Except for those already mentioned I enjoyed Stargate and The Arrival I think the name was as well.
Close encounter of the third kind, ET and Cocoon for feel good Sci Fi.

 

[appophylite]

I must agree with all the above responses....this question is too hard! I can't narrow my favorite sci-fi movies/series down by that much!
 
Incidentally, if my avatar doesn't lend a clue as to one movie that would certainly be on my list:
 
Serenity (under the assumption, as mentioned above, that one has the back-story from Firefly)
E.T.
Close Encounters of The Third Kind
Galaxy Quest
Dune (SciFi miniseries, if it can be counted as a movie 'series')
 
Not necessarily in that order and only marginally more favored than many others in my collection...

 

[balderon]

Although some consider the films campy I still enjoy many of the 50's "classics".
 
In no particular order...

1. Forbidden Planet
2. The Day The Earth Stood Still (1951)
3. War of the Worlds (1953)
4. Frankenstein
5. Star Wars (original trilogy)

 
EDIT: Honorable mentions: The Matrix, The Fly (1958), Star Trek Films (I, II, III)

 

[limpidglitch]

I've never been much of a fan of movies with space travel and that sort things.
I prefer it when the story is closer to current reality.
 
Delicatessen
Metropolis
Clockwork Orange
The Fly
The City of Lost Children

 

[joomongj]

1. Ghost In The Shell [1995]
2. The Matrix
3. Blade Runner
4. Minority Report
5. Predator

 

[wuwhere]

Forbidden Planet (I used to own a pair of Krell mono amps)
Blade Runner
Alien
The Terminator
The Matrix

 

[Uncle Erik]

2001
Blade Runner
Gattaca
Forbidden Planet
Moon

I don't think the 1950s movies are cheesy, well, not all of them. What they might lack in special effects is easily countered by story and depth.

 

[ZephyrSapphire]

Blade Runner
Tron
Ghost In The Shell
Akira
Alien

 

[kingtz]

In no particular order:

1. Donnie Darko
2. Matrix 1 and 2
3. Blade Runner
4. 2001
5. Alien 1
6. District-9

 
Other favorites: Terminator 1+2, Robocop, Predator 1, Star Trek movie (2009).
 
 
Edit: Added District-9.

 

[tjohnusa]

I would say:
Star Wars
The Matrix
The Brain That Wouldn't Die (50's camp)
Godzilla
Inception
 
Just off the top of my head

 

[SiriuslyCold]

wait, i missed Donnie Darko... that was Sci-fi?

 

[Jubei]

I'm sure I missed some great movies, but at the moment I'm thinking ...
 

1. Akira
2. Ghost In The Shell
3. Solaris
4. Rollerball (the original version with James Caan)
5. Matrix

 
Does Mad Max count as Sci Fi? Watch Dark City earlier today, and it was fun too.

 

[Landis]

Quote:

wait, i missed Donnie Darko... that was Sci-fi?

 
 

Spoiler

There's time travel in it, I believe it counts as borderline Sci-Fi.