Stargate-SG1 was great; though it should have ended at the end of season 8. Season 9 & 10 felt like some sort of odd reboot due to both the plot reset and character changes. That final battle over Antarctica between Anubis and the human forces was really the defining moment of the series. The series began with humans portrayed as these insignificant creatures, farmed for the amusement of godlike aliens, entirely powerless except for a few rare uprisings in which the aliens deemed them more trouble than they were worth. The battle at the end of season 7 portrays humans as having become not only powerful, but able to hold their own against these previously godlike aliens enough to disrupt the balance of power in the cosmos. Which would have been a really good note to end on for the series.
But instead, they brought in the two from Farscape and pulled an entirely new plot out of thin air. Mitchell's character was forgettable; he arrived out of nowhere, was suddenly in charge of SG-1, and in general wasn't anywhere nearly as interesting as O'Neil, whose character was very fleshed out from the beginning, and generally had more depth than Mitchell's generic soldier-boy routine. Valla's character suffered the opposite problem; she was interesting and fairly memorable, but really didn't fit with the show's premise and setting in the least. It was as if the character was written as a Farscape character, rather than a Stargate character. But I digress severely.
Stargate Atlantis started out pretty well, and had the advantage of having really good CG to make up for many of its shortcomings. The biggest one of which were the enemies suffering from a bad case of mook-disease. The wraith started out really badass; even the most skilled had a ton of trouble taking down one, with them regenerating to the point where bullets barely scratched them. There was even an entire episode in which a whole squad gets killed by a single wraith, whose regenerative powers are strong enough for them to run out of bullets, making them resort to shooting it from orbit with a spaceship to kill it. A couple seasons later, they're practically taking down hordes of wraith by tapping them on the arm. Though I don't think I watched the last season or two.
Then there was Universe. That was terrible. They tried to go the Battlestar Galactica route with the spaceship, which would have turned it into the same sort of character piece and examination of psychology. Problem was, nearly every character was boring as hell, with actual plot moving soooo slow. It had some CG pretties, but that was it. Firefly did more in its first episode than SGU did in its entire first season.
SGU episodes 1-3: "We fixed the O2 scrubbers."
SGU episodes 4-5: "The ship will recharge its batteries without us doing anything."
SGU episodes 6: "We found an ice planet and brought back what looks like 400 liters of water, (which somehow equated to refilling all the water supply we need for a couple hundred people, because fuck logic)."
SGU episode 7: "This episode takes place mostly on earth, because Spaceships and Stargates are boring. Also an attempt to get home which predictably fails because this is episode 7."
SGU episode 8: "Time shenanigans which in the end were almost entirely pointless."
SGU episode 9: "This episode takes place mostly on earth, because Spaceships and Stargates are boring. Time for a redneck marital problems plot!"
SGU episode 10: "In this episode, we do some boring stuff, but mostly set up for the actually interesting things."
Seriously; by episode 10, viewership dropped by a full 45% from the initial viewership number.