Zardoz, anyone?
Seriously though, 70-80s sci-fi films are the best, and I speak as someone who doesn't have any childhood memories of that stuff. I have childhood memories of Total Recall, though. Well, nightmares, mostly.
Among my favourites, apart from the obvious masterpieces like Alien, Blade Runner and Brazil(and everything else by Terry Gilliam as well), are:
Logan's Run, with it's adorably kinky 70s costumes and set-pieces and charming conceptual naivete;
A Boy And His Dog, featuring young Don Johnson and a telepathic dog in a post-apocalyptic world;
Mad Max and Mad Max 2 (or The Road Warrior) - classic post-apocalyptic action;
Soylent Green, Omega Man and Planet of the Apes - all featuring the delightfully horrendous overacting by Charlton Heston and tons of silliness, but some moments of pure sci-fi bliss;
Time After Time - featuring a time-hopping HG Wells, trying to catch
Irenicus Jack the Ripper;
The Man Who Fell To Earth, featuring even more David Bowie's junk than Labyrinth
;
Also - pretty much everything by Carpenter(Escape from New-York, Big Trouble In Little China and more) and Cronenberg (especially Videodrome) and... there's just too many to remember them all, and there's plenty of great sci-fi from the 90s too. Lucky us!
Fantasy genre seems to be slightly less fortunate in the great movies department, but Princess Bride, Labyrinth, Dark Crystal and Legend are all quite wonderful.
And there's a bucketload of amazing Hayao Miyazaki films, of course, with Nausicaa Of the Valley Of the Wind being my personal favourite.