Lot's of really great ones:

The Thing
Forbidden Planet
The Day The Earth Stood Still

To say nothing of classics like Earth vs. The Flying Saucers, The Blob, Cat Women On The Moon and Invasion of the Saucer Men.

And was Ray Harryhousen the John Browning of 50s special effects or what?

50 years later these films are still great. I grew up watching them when they were already 25 years old and they captivated me. I can still recall staying up late to watch these on TV along with the old Universal Horror classics.

I think the best one was probably "The Day the Earth Stood Still." It is just an impressive film, and the music score almost defines the genre all by itself. For pure "effects" probably nothing is as fun as "Earth vs. The Flying Saucers" and "Forbidden Planet" was an incredible story, but I still think they fall short of The Day the Earth Stood Still.

These were some fun movies.

About two decades too late for the 50s, but Colossus: The Forbin Project remains one of my favorite sci-fi flicks of all time.

The 50s had some great SciFi: War of the Worlds, The Incredible Shrinking Man, Invasion of the Body Snatchers...Nothing like Cold War tension to get some great movies made!

War of the Worlds

War of the Worlds and The Blob were two of my favorites.With Fordidden Planet thrown in for good measure.

Definately The Day the Earth Stood Still, I loved that movie, kinda odd for someone my age.

Forbiden Planet is next on the list for me also, a young Leslie Nealson, and one hell of a story.

I don't remember if it was fifties or very early sixties, but "THEM" was pretty good as well.

"The Crawling Eye" 1958 starring Forrest Tucker

"The Blob" 1958 Starring Steve McQueen

"The Thing From Another World" (AKA The Thing)  1951 starring James Arness (Matt Dillon) as "The Thing"

Destination Moon ranks right up there.

Conquest of Space while not a "great" film is pretty good too.

When Worlds Collide is memorable also.

Some great films are mentioned here, but The Day The Earth Stood Still is my favorite.