Most of my favourites have been mentioned here already and quite honestly there are very few sci-fi movies I won't watch and very few I do watch that I don't enjoy on some level.
I've scrolled through the thread though and I can't believe no has mentioned John Carpenter's
Dark Star - it's fucking brilliant!! Did I miss the mention - or have none of you seen it? Or am I alone in loving it, maybe? I mean it is very low tech (alien equals painted beachball with feet stuck on

) but the conversations with the bomb are some of the funniest moments in sci-fi movie history, if you ask me. And the end is bloody great too.
"Don't give me any of that intelligent life crap, just give me something I can blow up"



Dark Star (1974) - Seriously - check it out.
EDIT: But then I love
Mars Attacks too - so that might just be me (
Ack! Ack-ack-ack-ack-aaaaack!)