Again, what counts as science fiction to you?
Based upon the SFWA definition of Hard SF from the 1950's as quoth by an author in an intro...
Hard SF: No more than 3 breaks from known physics.
Soft SF: 4+ breaks from known physics. I can't remember the upper end cited; ISTR it being either 6 or 10, but can't remember the specifics.
The standards of Space Opera: FTL, artificial gravity, and ultra-high ISP (if not reactionless) drives put almost all space opera right out of the running for hard SF, and much of it well past that.
The Andromeda Strain is excellent hard SF, and still holds up.
Logan's Run, as well. The movie
Apollo 13 - there were fictionalizations for dramatic needs, but remains hard SF based closely on reality.
Iron Sky - in the soft end of SF. Banks' Culture series is on the soft SF side.
The Martian is arguably hard Sci Fi, but definitely Sci-Fi.
Silent Running is my favorite 70's Sci-Fi film.
Ethan of Athos is a great bit of Sci Fi in an otherwise mostly space opera setting (the Vorkosiverse).
Life Force.
Destination Moon.
Twilight: 2000 as well.
Just because it's not Sci-Fi doesn't make it bad. I've a deep love of the classics of Space Opera...
Lensman,
Star Trek,
Star Wars,
Buck Rogers (both the Buster Crabbe version and the Gil Gerard version),
Flash Gordon (including the Buster Crabbe version),
Alien,
oBSG,
Battle Beyond the Stars. Niven's
Known Space and Niven & Pournelle's
CoDoVerse.
I also like some techno-fantasy, such as
The Trigan Empire.
Krull was pretty fun, too. (just rewatched it.)