Destination Moon (Written by Robert Heinlein but heavily impacted by Hollywood even though Heinlein was the technical advisor.)
Earth Vs. the Flying Saucers - Great n special effects for a 1950s flick. FX by Ray Harryhause
The Time Machine - 1950s version.
The Cabinet of Dr. Caligari - great silent!
Alien - Great concept, great design. Loved the cat and have to admit I like Sigourney Weaver.
Flesh Gordon - Really guilty pleasure! No, that is the correct spelling... Campiest SF movie send up of all time. Helps that there are lots of topless girls. But the stop action "King Kong homage" monster's jaded commentary is great.
Close Encounters of the Third Kind (The original edition without the added ending.)
The War of the Worlds - George Pal version.
When Worlds Collide written by Phillip Wylie. (can you tell I like George Pal's stuff?)
Invaders from Mars (1953 version). Really creepy.
Invasion of the Body Snatchers - another creepy 1950s SF flick. The later two can't hold a candle to the first.
The Little Shop of Horros - the original. First movie with Jack Nicholson.
The Little Shop of Horrors - the musical version... Great fun... and Audrey (Ellen Green) is the sexiest and cutest thing on two legs. "Sure!?"
Somewhere in Time with Christopher Reeve... I like this because it was made where I went to College - Mackinac Island - and the movie theater where Reeve watched his love perform was the theater I was in charge of at college!
Honey, I shrunk the Kids - just great fun.
Forbidden Planet The Krell laboratory scenes were great and the saucer spacecraft was superb.
Quatermass and the Pit AKA Five Milion Years to Earth is one of the best Brittish SF films.
20,000 Leagues Under the Sea Disney version... fun.
The First Men in the Moon another Ray Harryhausen film with good humor... "He had such a terrible cold."
Monsters, Inc. Pixar's tour de force animation.
All three of the Back to the Future series.
Retroactive about the problems of trying to fix problems through time travel... perhaps the best time travel film.
Westworld with the original Terminator (Yul Brynner).
Robocop - Great satire.
Brazil - more great satire and a rogue appliance repairman as the protagonist.
Minority Report... I know, I know, its a Tom Cruise vehicle but it works.
The Abyss
"When Worlds Collide" -- I finally saw that movie early this year, when I saw it on DVD at the library. I really loved that scene during the landing when the untrained rookie rocket pilot said, "we've still got a quarter tank" of fuel. I almost rolled off the couch laughing. Still better than the original novel AFAIC. ;')
"Somewhere in Time" -- yeah, that's a good example. Premiere screening was in the Grand Hotel I believe, and when the sound track failed, Reeve stood up and filled in the dialogue for the audience until it was fixed.
I remember many a Saturday afternoon in the late fifties, early sixties, watching black and white sci fi films on a small TV screen: starships invading, mobs running thru the streets screaming. I'm tellin' ya, Saturday daytime TV just isn't what it used ta be!