I am not super high-falootin' or anything. I watch movies because they make me happy and/or enjoy the experience. Therefore, sometimes I don't resonate on these lists as much as some.
For example: Yes, I get that Alien was the genre bending film and is *superior*, but I watch Aliens far more, I find it more enjoyable. Especially if you get the good cut where the aliens are coming over underground in the tunnel and they deploy the remote MG's that just barely stop them.
So, Aliens and Alien.
The Thing. Second only to Big Trouble in Little China, this is a masterpiece for Carpenter/Russell. I was 11 or 12 when I saw it, and the couch scene had me jumping up and down.
Planet of the Apes (1968) I like them all, though, tbh.
I know everyone loves T2, but Terminator is my favorite.
Soylent Green
RoboCop is fun. It aged okay.
The Day the Earth Stood Still (1951)
Star Trek (OS: IV is my Fave, but II is awesome) Pretty much the even ones are amazing, the odd ones filler.
Predator GET TO THA CHOPPAH!
Star Wars IV and V
Inception
Interstellar - everyone should see it once.
5th Element - Multipass!
Close Encounters
Strange Days
Blade runner is as dystopian as it is Sci Fi, so since that ceiling is broken:
Mad Max, Road Runner
My Science Project
City Limits
And since someone mentioned Red Dwarf (I TOTALLY second that, btw)
Battlestar Galactica (original)
Battlestar Galactica (scifi channel one circa 2004)
Fringe
Dune (The Sci-FI Channel mini-series is my favorite, over the 1984 David Lynch film, sorry Sting, and the new ones).