"Star Trek"s 2, 4, 6, "Generations" and "First Contact"
"Star Wars" saga (though it never gets better than "TESB")
"War of the Worlds" (1950s original---I missed Tom Cruise's)
"The Matrix" (1)
"The Terminator" movies
"The Day the Earth Stood Still" (the orignal with Michael Rennie--didn't see the one with Keanu)
"Invasion of the Body Snatchers"---either version; they're both good for different reasons.
"Alien" and "Aliens"
"Fantastic Voyage"---Raquel Welch in that wetsuit!
"Close Encounters"
"ET"
And what the heck....I'm gonna add the "Back to the Future" movies....time travel, flying Deloreans and the flux capacitor are all sci-fi!