As you said, many of the finales listed were merely, as with Quantum Leap, Lois and Clark, and Scrubs, final episodes of a season of shows not renewed. I don’t fault them as much as the ones written as a series finale.
Almost as bad was my favorite series “Moonlighting” which took a basic episode then slapped a few minutes at the end to give the illusion of a finale. No finale at all would’ve been better. MacGyver did the same only worse with just a 30 second voiceover.
I’d list Star Trek Voyager’s finale as among the worst. It took six posthumous novels to really explore what it would have meant for the crew to return to their own Galaxy and the aftermath of everything they did to, for, and with the Borg.
Also on the list should be the entire final season of The West Wing, which bore little to no resemblance to the first several years of the series and offered only the most superficial character resolution… some characters were changed entirely to the point even the actors admit they were lost in how to portray them. Between that and the death of a cast member, the finale was basically the characters as mindless zombies.
I disliked the series Dinosaurs but for what the show was, the finale was superb and extremely daring for a show of that time.
The reboot of Battlestar Galactica was so poorly written that there was no hope that the series finale would be good, so it probably scores points for not being as bad as it could have been.
We’ve been rewatching House the last couple months (annoyed they didn’t secure the rights to the theme song for streaming distribution). Once the series had the “Survivor” season where House picked a new team, the series suffered a catastrophic decline. So although the finale was bad, by that point the whole series was bad.