Well besides the bespoke classics such as BladeRunner and The Man Who Fell To Earth, may I lighten the mood and suggest "Morons From Outer Space" (They came, They saw, They did a little shopping!) with Griff Rhys Jones, Mel Smith and Jimmy Nail? I only have it on good ol' LaserDisc but it's still brilliant.
Another classic is the Tim Allen vehicle "Galaxy Quest", with Sigorny Weaver, still cracks me up.
There's always the painfully funny "Coneheads" if you're really feeling silly.
Perhaps looking dated now, but "VideoDrome" with James Woods and Deb Harry is sorta sci-fi I guess.
I thought "Hitchhiker's Guide To The Galaxy" was a little weak compared to the book, but still quite a hoot.
Then "Mystery Science Theatre 3000" would also qualify if you could track it down. The concept of the MST series was simply but brilliant, a group of misfit aliens watching classic B&W scifi films and making hilarious comments a la Beavis and Buthead.
You will either love it or hate it but I think it's scream funny.
Honourable mentions go to "Howard The Duck" from George Lucas (another LaserDisc), "Mars Attacks!" by Tim Burton is another great waste of time, as is the "Idiocracy" by the guys who are behind South Park.
Shortest and funniest scifi romp has to be Steve Oderkirk's brilliant "ThumbWars (The Phantom Cuticle)".
There's a whole series of his twisted, genius parodies in my collection including GodThumb, FrankenThumb, ThumbTanic, BatThumb, BlairThumb Project, etc.
I won't spoil the big reveal at the end of ThumbWars, but when Darth Vader rips off his cape to reveal a pink ballerina outfit and coming out as Luke's mother, then you know it's going to get silly.
Princess Lea has sticky buns on the side of her head, for crying out loud!
I'd just got out of hospital and someone got me the discs from the US to cheer me up and I swear I almost burst my stitches laughing so much. I had to watch them a bit at a time as it hurt so bad.
But for down right serious scariness, solid believable storylines without over the top laser battles, CGI overindulgence or questionable rubber monsters, it's gotta be "Bladerunner" (5 disc Blu Ray edition) and "The Man Who Fell To Earth" with Bowie (Criterion DVD import pressing).
HiDef greets also go out to "Matrix" not forgetting "Fifth Element" and "Brazil".
SciFi fans who enjoy a touch of humourous splatter fare can do no better than "Shaun Of The Dead".