I've read through page 13 so far... love every movie listed that I saw. Below are a few of my takes, and yeah, too hard to pick only 5:
Top 5 or 10 or something:
Star Wars - awesome. Shaped half my childhood.
Godzilla/Gojira - Shaped the other half of my childhood. the original is dark and deep. Loved all the 60s and 70s ones as a kid. Still have a soft spot for all the ones since.
Alien - scary good.
Aliens - action good.
Predator - action fun.
Blade Runner - interesting, unique, started something. Yep, Hauser was just too good in this. The zoom in on a picture trope bothered me so much the first couple of times I watched it... funny that (usually BS) trope has been done so many times. You can almost imagine some digital photo where the data is there to zoom in nowadays, though.
Vampire Hunter D -> is it scifi, it has robot horses and cyber-fantasy type stuff? Probably more fantasy/horror. The Original movie was so unique for a hand drawn movie. The 2nd one looks incredible. Oh, I see the 2nd one Bloodlust is on someone's list, nice.
The Matrix - mind blown from the original trailer, the movie was so awesome back then.
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Honorable and/or had to comment on other folks lists:
Dune - 1984, was so cool as a kid. The worms, the fighting. I hadn't read the book(s) at that point.
Dune - the newer one, I really enjoyed it. Read the first book or two (first book is incredible and so incredibly influential) around that time, also.
Moon - great concept, love Sam Rockwell, that crazy guy makes every movie fun. Ah ha, I see it made another list!
Sunshine - was just great scifi, great cast.
Ghost in the Shell - thought provoking and wild.
Empire Strikes Back
Return of the Jedi
Mad Max (all of them through Fury Road) - so unique, a million copycats never could do it as well.
Terminator 1/2 - incredible.
Black Hole - definitely hit hard as a kid
Trek movies - several of them are great
Planet of the Apes (Heston version) - yep, that hit hard as a kid, too.
Back to the Future - the first one especially was just so much fun.
Primer - truly an incredible concept
Jurassic Park - how could I forget. The only thing wrong with it was I wish I could have seen it before puberty for even more of an effect.
Under the Skin - oh yeah, Older Beholder's comments are spot on. Original, disturbing, astonishing. I listen to the soundtrack every now and then... it gives me the same feeling as that movie every time.
Dark City - oh yeah, that one has influenced so many things after it (Inception, anyone?). Phenominal movie.
Love the B-list by The Soloist around page 9 or so, nice choices.