I think my all time favorite series finale is the last episode of Beavis and Butt-Head. While most of the episode was just a clipshow, there were some genuinely funny moments, like Stewart's flashback (THAT WOULD BE SWELL! UH-HUH-HUH-HUH!) and
Spoiler Below Principal McVicker having a heart attack when he finds out Beavis and Butt-Head aren't dead.
It was perfect right down to the end credits.
Another all time favorite is the Home Movies series finale. At first you're not sure whether it's supposed to be the last episode or not but during the last few minutes it becomes clear that the show is ending and it's never coming back. I think one of the most heart-wrenching moments is when
Spoiler Below Brendon accidentaly drops his camera out of a moving car and smashes it
Sad
As for the worst, I'd say St. Elsewhere. If you've seen it, you know what I'm talking about.