Rule of thumb: you generally get one good sequel and then the franchise falls apart IMO. And that was even before Hollywood started remaking and rebooting franchises with a nihilistic eye toward discrediting everything that made them great, and deliberately humiliating the fanbase in the process.
(*koff* Star Wars *koff*)
So, with that in mind:
Alien and Aliens. You can forget about the whole rest of that franchise.
The Terminator and T2: Judgement Day. Again, you can forget about the whole rest of that franchise.
Firefly (TV series) and the Serenity movie. Yes, I'm a proud Browncoat.
Babylon 5. One of the best TV sci-fi series ever, from beginning to end.
Original Star Trek plus STII: The Wrath of Khan. I've come to dislike most of the other Star Trek films and ST:NG (although Voyager and Deep Space Nine weren't bad), and flat-out loathe the JJ Abrams reboots.
The original three Star Wars films are a cherished part of my childhood. I'd add the Rogue One prequel and Timothy Zahn's Thrawn novels (particularly the original Thrawn trilogy that opened with Heir to the Empire; in which Zahn did a great job of capturing the magic of the original trilogy). With the rest . . . well, George Lucas lost the plot with the prequel trilogy; then Disney systematically murdered the franchise, abused its corpse and mocked the fans with woke BS.
Classic The X-Files.
The classic 80s show "V." Somebody mentioned "To Serve Man" back up the thread? Well, that was pretty much the series. And Jane Badler was campily, evilly awesome as the head lizard.
The Charlton Heston classics Soylent Green and The Omega Man. Heston at his scenery-chewing best.
The original Predator (and Predator II wasn't bad).
The Doctor Who spinoff Torchwood, which was Doctor Who for adults with far more complex and intriguing plotlines and relationships. (I never was able to get into Doctor Who, even before they woke-i-fied it.)
The original Robocop.
Event Horizon. Great blend of horror and sci-fi.
And while I'd love to see a remake of Starship Troopers that was faithful to the original novel and took its themes and concepts seriously (and gave us the powered armor!) . . . the film version is still hilarious. I mean, how can you not laugh at Feldmarshal Doogie Howser?