Inspired by the DVD I’m watching tonight: “X The Unknown”.
Fun flick, but my choice is “Earth Vs The Flying Saucers”.
Invasion of the Body Snatchers
Forbidden Planet
It Conquered the World, by Roger Corman.
It didn’t.
aka Invasion of the Flying Saucers, which explains why I always get that one mixed up with Invasion of the Saucer Men, with Frank Gorshin. Saucer Men was sorta schizo. If I remember right, the aliens injected something that either made you silly drunk, or it killed you – maybe both. It was funny and then people started dying. Or did they just pass out and people thought they were dead? I need to watch that one again.
It’s hard to choose between Body Snatchers and Forbidden Planet. Both of them gave me nightmares.
‘And we thought they were mad at the car!’
I’ve seen it in 3D! I’d put it as above average for 50’s SciFi low budget films.
I’ve got to pick the original Invasion of the Body Snatchers. I think it holds up best of all of the 1956 crop.
Another vote for * The Forbidden Planet,* the original Star Trek pilot. 
I learned too late that man is a feeling creature…
I’m gonna go with Invasion of the Body Snatchers. But let’s not forget our British friends. How about a shout-out for The Quatermass Experiment (a/k/a The Creeping Unknown), released in the US in '56.
Cool. I was born in '56. Next birthday I’m going to have to watch these.
I was going to say Sincerely Yours starring Liberace as a piano-playing playboy, but was disappointed to learn that was released in '55.
That would be the 1956 classic Amazon Women on the Moon. I haven’t managed to see it all the way through yet without interruption, though. 
Bill Door got it – Forbidden Planet is not only my favorite SF flick of 1956, but on alternate days it’s my favorite science fiction film of all time. On the other days it’s 2001.
Well, if we’re making a list, let’s not forget Godzilla, fer cryin’ out loud.
And then there’s the classic B-movie (wait, is that possible?) The Mole People.
(Or don’t science-fictiony monster movies count?)
Hard choice to make, real world paranoia or unreal world psychosis?
Edit; Now I see Godzilla is up there too, hard choices!
Found another one, which I haven’t seen, but which gets a pretty good rating (6.2) at IMDb: X: The Unknown.
Hmm. Anyone seen this one?
You mean besides the OP?
I’ve seen it plenty of times. I wasted my younger years consuming mass quyantities of sf and horror films on TV.
Interesting film, but not particularly notable. One of many “blob” films on the fifties and sixties.
Day of the Triffids
The Midwich Cuckoos/Village of the Damned
Lot of good ones, to for me Forbidden Planet blows the rest out of the water.