Well, I have to mention the first ever, the amazingly weird A Trip To The Moon 1902 by Meliès. Crazy one.
La Jetée 1962 of which 12 Monkeys is a remake.
Sleeper 1973 Woody Allen at his best. I want an Orgasmatron.
Things To Come 1936
This Island Earth 1955
20 Million Miles to Earth 1957 This Ymir thing is wild.
Carpenter's Dark Star 1974
The Invisible Man 1933
It Came From Outer Space 1953
Invaders From Mars 1953
One of the best EVER, the great Farenheit 451 1966
And OF COURSE Barbarella 1968
Those are some of my old school favorites.
But azathoth's lists are great too.
Man, I freaking LOOOoooVE Barbarella...