Quick, what series holds the title of longest running American SciFi series? Nope, it's not ST:The Next Generation, Voyager or any of the other Trek iterations. Curious? Well, way back in 1994, Director Roland Emmerich (Independence Day, The Patriot, 2012) helmed a movie called Stargate. It starred Kurt Russell, James Spader and Jaye Davidson, fresh off his big role in The Crying Game. The film was a solid success, earning nearly $200 million worldwide, a good sum for the early 90's. Though it was a 'hit' movie, it would be its TV descendants that would leave the bigger mark.
If you never saw the movie, it was centered on an archeologist named Daniel Jackson who was quickly becoming a laughingstock for claiming that the great pyramids were landing platforms for aliens. After a particularly bad lecture he is offered a mysterious job on a US military project. He finds himself trying to decipher symbols on a large ring shaped artifact that had been discovered in Egypt back in 1928. His breakthrough allows them to activate the 'Stargate' which promptly opens a wormhole connection to another identical gate on another planet. The movie plot takes off from there and, as you would expect, vindicates Jackson's theories and action, romance and special effects abound.
As successful as the movie was, no sequel ever materialized and in 1997 the story was revived as a Showtime series as Stargate: SG-1. It would, after season 5, move to the SciFi Channel for the rest of its run. They took the general plot and made a few tweaks that opened up the 'universe' a bit and allowed the kind of freedom a series would need to thrive. Surprisingly though, the vast majority of the structure remained, including many of the core characters, though all but a couple supporting actors were recast. The updated storyline had the secret project becoming 'Stargate Command,' a super secret operation that was run from twenty some odd levels beneath Cheyenne Mountain, Colorado. The 'SG-1' of the title refers to the first, and most elite of the Stargate teams made up of the two primary characters from the movie, Dr. Jackson and Air Force Colonel Jack O'Neill along with two newly created characters. Captain Samantha Carter, an Air Force officer and astrophysicist, and T'ealc, an 'alien' who joins the team following the series pilot.
Much like the way Star Trek: TNG continued for a few more seasons after Deep Space 9 debuted, SG-1 still had two seasons ahead of it when they spun off Stargate: Atlantis. Atlantis was based on an alien city located far off in the Pegasus galaxy and actually spent its first season cut off from Earth. Atlantis ran for five seasons before being shuttered to make way for Stargate: Universe. Universe took place on an alien starship that was on an automated, unmanned exploration mission far off in the universe. The humans arrived onboard, via stargate, while escaping disaster and becoming marooned on the ship with minimal supplies and limited understanding of the ship and technology. Universe only managed two seasons before being cancelled. There was also a short lived animated series called Stargate: Infinity, which I never saw.
Both SG-1 and Atlantis held to a similar, successful formula that focused on a small group of interesting and likable characters exploring and often fighting for their survival as well as the survival of Earth itself on occasion. Universe was a completely different beast and I believe that's what spelled its doom. Whereas SG-1 and Atlantis were both adventure series, spiced with humor and sprinkled with a bit of drama, Universe was a heavy drama spiced with adventure and an occasional pinch of humor. I was only able to manage about half a season before I gave up on it. Personally I found it way too dark and brooding where every character was deeply flawed and there seemed no real good guys. Think 'Stargate: Galactica'. I much preferred the lighter tone of the first two series. Both SG-1 and Atlantis had some interesting characters with their own quirks that were fun to watch. The writing was very good and the special effects were excellent, especially for cable TV show. Even the various sets were much better than you might expect. Another thing that set the first two iterations apart from some other science fiction series was that they weren't above taking gentle shots at sci/fi in general and Stargate in particular from time to time. For example a later episode of SG-1 had one character comment about a really corny line, a line that that character herself had delivered verbatim in one of the first episodes of the series.
If you haven't seen Stargate: SG-1 or Stargate: Atlantis, or if you never really gave them a chance when they were in production, I highly recommend checking them out on Netflix sometime. Both series are available via disc and streaming. The first few episodes of SG-1 are a little rough in places, like pretty much every sci/fi series, but it quickly settles in as a top notch bit of science fiction. They may not have gotten as much attention as Star Trek, Babylon 5 or others, but SG-1 and Atlantis are well worth your time!