My own nominees are:
1. Day the Earth Stood Still
2. War of the worlds
3. Start Trek
4. Star Wars
5. Close Encounters of the 3rd Kind
I'm not a great movie person. I don't keep a lot of movie titles in my head. I rarely watch movies. I do like Sci Fi but I'm very picky. There's a lot of trash in the genre.
Though I do fondly remember the TV series LOST IN SPACE & MY FAVORITE MARTIAN.
I don't think I ever did watch that one about the bloke alone on Mars.
I did like the one about the huge artificial enclosed environment above the earth. Elysium?
I enjoyed some of Edgar R Burroughs' 'Mars books.'
And STRANGER IN A STRANGE LAND
And, ENDER'S GAME series
I vaguely recall that there's a bunch of dystopian society movies--a few of which I watched. I find them usually pretty dreary. Not a favorite use of my time.
Anyway--that's about all that's off the top of my head currently.