Among others not previously mentioned that come to mind:
Cosmic Journey (1936)
Incredible Russian flick is a VFX extravaganza boasting fantastic miniatures and (some) mindboggling stop motion depicting a trip to the moon, baby!
Clips - https://www.youtube.com/watch?v=MqpKd70rZOs
The Invisible Ray (1936)
Karloff and Lugosi in the story of Radium X. “Janos! No!”
The Man from Planet X (1951)
I think this was the first Hollywood film with an extraterrestrial, beating* The Day the Earth Stood Still* to release by several months. Low-budget and saddled with some silly (fake-)Scottish sheepherders, but the alien has a unique (paper mache) look and overall it’s a serious and far less pretentious film than the better known Robert Wise one.
Unknown World (1951)
Scientists use the ultra-cool Cyclotram to find a new world in the center of the Earth.
The Magnetic Monster (1953)
A different kind of monster movie with a spectacular climax lifted from the German film Gold (1934).
This Island Earth (1955)
Good story, fine execution, some awesome VFX. This movie reveals why you can’t get good interocitors anymore these days.
Earth vs. the Flying Saucers (1956)
Classic sci-fi. “Look to your sun for a warning!"
Another unconventional monster movie.
*Twenty Million Miles to Earth *(1957)
Ymir comes to Earth from Venus and humans totally fuck with him. Illegal immigration-based parable.
The Mysterians (1957)
Seminal Toho-produced sci-fi.
The Crawling Eye (1958)
Aliens in the Swiss mountains make the mistake of messing with (“only you can prevent…”) Forrest Tucker and his legendary large package. Well-paced and suspenseful with a shot or two of stop motion giant tentacled eyeballs (yeahhhhh!)
The Atomic Submarine (1959)
Alien design subsequently used on The Simpsons.
Creation of the Humanoids (1962)
Ridiculously low-budget and cheesy-looking, yet incongruously blessed with unusually intelligent script delving into Blade Runner territory.
Wild, Wild Planet (1966)
Dr. Nurmi’s experiments prove controversial.
Five Million Years to Earth (1967)
Alien corpse in the London subway ends up inconveniencing thousands.
Latitude Zero (1969)
Possibly the most entertaining film ever made.
The Final Programme (1973)
Possibly still ahead of its time.