Must watch classic SciFi movies?

Some that I'm not seeing here much yet:
Hitchhiker's Guide To The Galaxy, War of the Worlds, Minority Report, Gattaca, Heavy Metal, Repo Man, Starship Troopers, The Fifth Element, Tron, Tron Legacy, Independence Day, Idiocracy, Predator, Star Trek (2009), Escape From New York, Galaxy Quest, Total Recall, Close Encounters Of The Third Kind, E.T., Wall-E, Jurassic Park (1993), District 9, Robocop, Children of Men, Inception, and the Back To The Future trilogy,

I will contribute one. The Last Starfighter. One of the first movies with actual CG. I recommend checking it out.

Forbidden Planet, Logan's Run, Death Race 2000, The Day the Earth Stood Still, ...

What's your cutoff for "classic"? Are we talking about the classic film era, or anything older than 25 years?

If you can handle subtitles Tarkovsky's 1972 version of Solaris is brilliant.

Here are some classic SF going back a bit further than most of the comments co far.

1970s

Invasion of the Body Snatchers (1978)

A Clockwork Orange (1971)

Westworld (1973)

Silent Running (1972)

Logan's Run (1976) - Overpopulation (the end)

Mad Max (1979)

Capricorn One (1977) - The movie that started the moonlanding was fake conspiracy.

Soylent Green (1973) - Overpopulation (the start)

Colossus: The Forbin Project (1970) - AI taking over the world.

The Andromeda Strain (1971) - Pandemic

Rollerball (1975)

Sleeper (1973)

Slaughterhouse-Five (1972)

1960s

2001: A Space Odyssey (1968)

Barbarella (1968)

Fahrenheit 451 (1966)

Fantastic Voyage (1966)

The Time Machine (1960)

Robinson Crusoe on Mars (1964)

Journey to the Far Side of the Sun (1969)

1950s

The War of the Worlds (1953)

When Worlds Collide (1951)

The Day the Earth Stood Still (1951)

Forbidden Planet (1956)

Invasion of the Body Snatchers (1956) again bit I prefer the 70s remake.,

Plan 9 from Outer Space (1957) The first so bad it is good movie.

On the Beach (1959)

Destination Moon (1950)

Also, if you count ~20 year old movies as classic, I’d add Bicentennial man and Artificial intelligence, haven’t seen these mentioned yet.

A lot of great ones out there but my personal favs probably are Brazil, 12 Monkeys, Fifth Element, Event Horizon, Blade Runner in the “earnest” category, then Tokyo Gore Police, Johnny Mnemonic, Starship Troopers, Tetsuo, Total Recall, RoboCop in the campy/wild/fun category.

I noticed a lack of Anime. If I may:

Ghost in the Shell (the entire collection if you have the time)
Macross: Do You Remember Love?
Akira
Galaxy Express 999
Captain Harlock
Battle Angel Alita
Serial Experiments Lain (Series)
BubbleGum Crisis Tokyo 2040 (Series)
Red Baron (Series)
Saber Marionette (Any, but J is a series that hits maturity in the second half with aplomb)
Neon Genesis Evangelion (Series)
Cowboy Bebop (Series)
The Big O (Series)
Roujin Z
Final Fantasy VII: Advent Children
Iria: Zeiram the Animation
Steamboy
Paprika

And more I can’t think of right now.

Time Crimes (2007)

9 (2009)

Pandorum (2009)

One of my favorites that seems to get overlooked a lot lately is Contact.

Golden Age (70s & 80s): Blade Runner, 2001, Close Encounters of the Third Kind, Alien, Wrath of Khan, Undiscovered Country, E.T., Brazil, Lynch's Dune, Back to the Future, The Thing, They Live, Akira, Buckaroo Banzai, The Abyss

90s Renaissance: Total Recall, 12 Monkeys, Contact, Fifth Element, Starship Troopers, Galaxy Quest, Truman Show, Dark City, The Matrix, Gattaca, π, Strange Days, T2, First Contact, Event Horizon, Jurassic Park

I guess most of the famous classics are going to be listed anyway, so to add something that most people probably haven’t seen, I suggest *batteries not included. It’s nothing grandiose (more like a family-friendly flick, I feel like it’s in the same category as Flubber or Short Circuit), but it’s really sweet, light-hearted and enjoyable.

Gonna just list the ones that really jumped out to me:
Event Horizon; Bladerunner & Bladerunner 2049; Pacific Rim; Johnny Mnemonic; Stalker; Solaris (1972); Robocop; Starship Troopers; Dredd; Tetsuo the Iron Man; Crimes of the Future; Pandorum; Fifth Element; Southland Tales

I didn’t see it on anyone’s list so far but I would recommend The Andromeda Strain (the original).

Nineteen Eighty-Four (1984)

My all time fav is The Thing https://letterboxd.com/film/the-thing/
It's more mystery than sci-fi I'd say.

Firefly (Is it old enough to be a classic?) is a must watch IMO.

It's more fantasy, but it has a lot of sci-fi elements (IMO): Angel’s Egg https://letterboxd.com/film/angels-egg/
Apparently, I don't know if true, it was an inspiration for some Fromsoft games like Dark Souls. It does have a quite dark and meditative mood.

Dark City, 2001 a space Odyssey, 2010 the year we make contact, enter the void

@MooseGas Fantastic Planet is an old classic French animated flick that's quite interesting.

"The Quiet Earth" definitely worth watching.

Fantastic Planet

Ghostbusters!

Star Wars Ep 4

Star Trek 4

The Thing (1982)

Aliens

Terminator 2

Ooh, I have a list:

• Looker - directed by Michael "Jurassic Park" Chricton. The first ever film to create 3D shading with a computer that produced the first ever CGI human character was the model Cindy (Susan Dey).
• Westworld (1973) - also written by Michael Chricton
• Mac and Me - It's a movie that's SO UNBELIEVEABLY BAD IT MUST BE SEEN TO UNDERSTAND HOW BAD IT IS. After you watch it, you'll understand why Paul Rudd tortured Conan O'Brien with this famous clip.
• The Last Starfighter - another cheeseball 80s knock off of Star Wars. Also pioneered the use of CGI
• Enemy Mine - Dennis Quaid bonds with an alien enemy and eventually helps him bear children. Directed by Wolfgang 'Neverending Story' Petersen. You have been warned.
• The Quiet Earth - A scientist awakens to find himself alone in the world. In a desperate attempt to search for others, he finds only two who have their own agenda.
• The Stuff - A delicious, mysterious goo that oozes from the earth is marketed as the newest dessert sensation, but the tasty treat rots more than teeth when zombie-like snackers who only want to consume more of the strange substance at any cost begin infesting the world. Co-stars Paul "Goodfellas" Sorvino
• Miracle Mile - What would you do if you accidentally found out that the world was ending in a little over an hour? Features Anthony "ER" Edwards

Lol at Zardoz being on anyone's "must see" list.

Arrival
The Martian
Interstellar
2001
Bladerunner (& 2049)
Sunshine
Ex Machina

Kind of surprised I didn't see anyone mention Flash Gordon, from the early 80s. It's not a great movie, but it's highly entertaining and Queen did the soundtrack.