To me, REAL science fiction movies are rarer than hens teeth. Most end up being in the realm of "science fantasy" more than science fiction. But, I'll give it a shot... (in more or less chronological order)
1. Der Frau in Du Mond: (1929) Fritz Lang. When you realize just how long ago this movie was shot, even before the Great Depression, you have to marvel at just how accurate it was in it's depiction of space travel. Unfortunatley, Mr. Lang blows it in the final reels, by suggesting there is air on the moon.
2. Things to Come (1936)
3. Destination Moon: George Pal. Again, an attempt at creating a scientifically accurate deprction of space travel.
4. The Day the Earth Stood Still (1951) For it's time, it was real science fiction. Given the one fantastic element, the arrival of an advanced alien on Erath, pretty much everything is plausible.
5. Forbidden Planet. (1956) This one is a boarderline case. It really depends on the invention of the "hyperdrive"
6. 2001: A Space Odessy.
7. The Andromeda Strain.
9. THX 1138
10. Solaris (2004) Again, a boarderline film, but it's a personal favorite.
Number Two: Conform, Number Six! Conform!
Number Six: I will not be stamped, filed, indexed, briefed, debriefed, or numbered! I am a person.