Close Encounters of the Third Kind it always intrigued me as a kid, ships and planes that have been missing for years suddenly and with out warning appeared throughout the planet and communicating using light and sound just amazed me back in the 70's....

Firefly and Serenity will forever be a cult classic.

I watched Serenity before Firefly - didn't ruin it for me.

Oh yeah, forgot to add Weird Science and Judge Dredd for my fav movies.

I've always liked Demolition Man over Judge Dredd. There's just something about the goofy state of society in that movie that makes me smile.  It's like the farthest left on the political spectrum of San Francisco took over society and banned anything remotely bad, or that could offend even one person. 

That and I wonder how anyone is supposed to use those three sea shells. 

Uh, this is difficult

I wholeheartedly agree with the Children of Men sentiment, its brilliant movie, i saw it only about year or 2 ago, before i kind of resisted to it, as i did not like the story that much, well no spaceships, aliens, mystery, nothing interesting, .... but it was mistake, it was great, though in its own way. BTW i really really loved music in that movie, there was very tender cover version of Ruby Tuesday, The Court of the Crimson King, Bring on the Lucie from John Lennon and beautiful and meaningful song called There is an ocean by singer called Donovan (i never heard about this guy before)...easily one of the best movie OSTs out there...

Other Sci-fi´s i liked:

The Fountain

Gattaca

Minority Report

Impostor

Contact

Abbyss

A.I.

Dark City

Matrix first movie

Serenity

etc...

Funny thing though, while all these are great and many others, there is no such sci-fi movie, i would love to see: something set in future and space, something revolving around some kind of mystery, featuring aliens, spaceships and some drama, basically something along the lines of Star Trek or Babylon 5, but obviously not Star Trek or Babylon 5. You have either these sci-fi dramas - movies with serious plot set in near future, or shallow all action flicks like Independence Day, this new Battle Los Angeles, Skyline etc...but nothing fits the criteria i mentioned.

I would really love to see Conflict Freespace or Homeworld movie.

+1 for Dark City, Contact, Event Horizon, Predator 1 and 2 (exclude Predators at all cost !!!), Alien (all) (exclude AVP 1, 2nd was fun), Terminator (exclude Terminator 3), K-Pax (I don't know if we can say it's a sci-fi movie but I definitely love it !!!! One of the best indeed).

+1 for H2G2, where I read the books too and it's awesome !!! Big fan of it ! Especially when they say "Belgium" for no special reason actually (since I'm from Belgium but live in USA now)

+1 for Titan A.E. it's an animated movie but I definitely loved it !!!, we could add the treasure planet too !

As for Avatar (which really is Pocahontas in space), I liked it ! But I think it would have been way better if at the end we would have seen the bad guy smoking a big cigar, asking to launch a nuclear bomb on them blue guys, and he would throw his cigar in the air, there would be a slow motion scenes where the camera follows the slow fall of the cigar and when it hit the grounds, the camera would change back on the planet and you would see that marvelous nuclear explosion ! Camera comes back on the bad guy saying something like "yeaaaah" then black screen, credits with epic music !!!

Well, so many great sci-fi movie....

But Definitely, my favorite one are the Dune movie (old and the 2 new ones which are more like a mini-serie)

Reading through the list after posting mine, I really did not see any that I missed in my top 5.  All the ones mentioned are good, and we each have different tastes, so it is good to see such variety.

But if I had to pick a #6, I was reminded by some of Children of Men.  Another great one IMHO.

Well, it's hard for me to pick a favorite so I'll just list a few that I really like, in no particular order:

The Matrix (original - hated the sequels)

Serenity

2001 A Space Odyssy

Planet of the Apes (original)

All of the Star Trek films

Homeworld, particularly the original, is so perfect for a movie, and now that the IP for it has returned to Hiigara, it could be possible.

I like 'em both about the same.  They both had Rob Schneider in them too.  Also another movie I can't believe I forgot is Moon with Sam Rockwell.  Very well made, but would have liked to see an epilogue that took place on Earth.

American Godzilla... obviously 

THIS. This is something that would be awesome.

There are a couple of made for TV movies that I liked that I should mention -- not sure if they are among my favorites, but they are definitely worth watching:

"The Lathe of Heaven" (the original made for PBS, not the A&E remake)

"Between Time and Timbuktu" -- I would LOVE to find a DVD of this, and I've looked all over the place for it, but I'm not sure it actually exists. Basically, it was a movie made for PBS ages ago that kind of combined scenes, themes, concepts and characters from the works of Kurt Vonnegut.  Plus, it had Bob and Ray playing news anchors (I loved Bob and Ray).  I love the books of Kurt Vonnegut, which is partly why I loved the movie.

I actually once owned the published script from the movie, but I read it so often that the book literally fell apart.

And while I'm on the subject, I don't think anyone has mentioned "Slaughterhouse-Five".  The book, of course, is much better, but it's not a bad translation (and it's not science fiction, but there was also a decent movie made of "Mother Night" with Nick Nolte, which never got any kind of notice whatsoever.  It's not a great movie, and as I said, it's not Science Fiction.   But if you love the book like I do, or love Vonnegut in general, you should see it.

Oh i see now, somebody mentioned K-Pax and Event Horizon, i completely forgot about them, they were indeed brilliant....

K-Pax again does boast very good OST by the way and EH does have very compelling and interesting plot IMHO, for me personally it is far better combination of sci-fi theme and horror than Alien (though great itself) ever was...

BTW while its not sci-fi, (but these days judging by the SyFy channel it does not matter :D) its rather mysterious, i need to bring up the Mothman Prophecies movie with Richard Gere. Have you seen that? Its fantastic IMHO, for me the movie with most unsettling atmosphere...you can see horrors, where you are scared more in the particular moment, but this one makes you feel uncomfortable even few months after you saw it.

The mothman prophecy wasn't bad, I agree   as soon as I saw the movie I went over the internet to search for mothman stuff... found a lot of interesting stuff and learned about what it really is too, lol.

Event Horizon is definitely a must see, the only movie which reminds me of Space Hulk...

Oh--excellent 2-part made for TV movie that I think is way better than given credit for: The Lost Room

One of the few times Scifi created something actually really good (Epoch was not bad for them either but it wasn't an "A" movie).

I just came back from taking my son to a "Invadercon" for Invader Zim's 10th year anniversery.  We got to meet the cast and writers--they are all huge scifi geeks and consider Zim "comedic science fiction".  Originally there was to be another season and then a cinematic movie release.  Watch "Rubber Piggy" and "Dark Harvest" if you have never seen it.  Very funny even on an adult level.

Now if only Zim would kill Sponge Bob...

I think the first Epoch was (perhaps not in the strictest sense, but from a good movie perspective).  The sequels were definitely B.

War of the Worlds (1950)

Sifi Dune

Serenity

The Fifth Element 

Independence day 

Equilibrium  

ONLY the first--the rest were horrible. The original was actually worth watching with only a few "cringe at the cheapness" scenes.  Really interesting plot.  Great scifi story.

I just watched Tron: Legacy and am impressed.  Way deeper movie than I expected.  Wish they had more time and the audience venue to have gone deeper. 

You could see shades of the Matrix and Neuromancer all through it.  Wish I had seen the IMAX version now.  Darn.

And don't forget WWE wrestling! I hear it's actually "Syfy" nowadays.

I liked it as well.  But what struck me (and I know they did it on purpose) was how much it looked like the original.  In other words, while the art of computer animation has come a long way in the last 25-30 years, nothing really changed for Legacy.

I still don't understand why stuff like wrestling is on Syfy, it makes no sense whatsoever.

The Matrix trilogy

IRobot

WALL-E

Still a kid at heart

Tron is actually not so silly an idea with the tech being developed right now.  They have had a cap for years that can be placed on the head and allow you to move a cursor--now they have direct contact to the brain sensors that can "read" thewords you think and allow you to controlo a mouse and execute computer commands  A little further and they'll be able to make you see and hear things (in fact, they already can to a degree).

What I thought was brilliant about the movie was the "infomorphs"...spontaneously created, random, individual electronic consciousness'.  That is the actual holy grail of people working in the AI field.  The current goal is to build a program that qualifies as "sentient" but the ultimate goal is to build a program that can grow itself independently of its programmers--exactly what the infomorphs were.

So "Tron" was the creator's AI and the infomorphs were TRON's creation/children.

Admittedly, he would basically have to have been Galileo, Steven Hawking, Edison, Tesla, Einstein and Niels Bohr all rolled into one to do what he did in Tron.

I liked IRobot but lament how far short of the book(s) they fell on so many levels.