A total experience.
If I had to name just one favourite movie, it would be 'Forbidden Planet'. The expression hadn't been invented then, but it definitely 'blew my mind'! Until then, SF films had been about men (usually) landing on other planets or monsters coming to Earth. Here we come out of hyperdrive near the planet Altair IV. Superb.
I saw this in 1956, the year it was released, and I was just about to be demobbed from the RAF, at the age of 20.
I think I just saw a poster advertising it (a giant robot carrying a scantily-clad girl); how could I not go to see it? :-) But it exceeded all my expectations: wonderful SFX, weird electronic music, a story based on Shakespeare.. . I was already producing space art, and this only inspired me to further heights.
'War of the Worlds' was a favourite.
George Pal produced the first SF film I saw, 'Destination Moon', and used the work of the greatest space artist, Chesley Bonestell. He went on to produce 'When Worlds Collide', War of the Worlds', 'Conquest of Space', and 'The Time Machine'.