So, which series finale(s) would you put on such a list?
For me, at the top of the list would be the final episode of Star Trek: The Next Generation, "All Good Things". It really brought the series full-circle, and made it seem like they'd planned it that way from the beginning, even though they were pretty much just making stuff up as they went along. It does have some plot holes in it, but pretty much any story having to do with time travel does. And it did a good job of showing how much the characters had changed since the first episode, even Q.
Homicide: The Movie
Even though it was billed as a made-for-TV movie, it was made to give closure to all the characters' stories from the series, especially Gee, Pembleton and Bayliss.
And now I'm really going to stretch the boundaries of what could be considered a series finale
MST3k: Laserblast
Okay, technically it wasn't the series finale, but for some reason, I liked it a lot better than the actual series finale. I don't know, I guess it was just the fact that it seemed like there was more of a mood of "Hey, this may be our last episode ever, so let's just get out there and give it our all and have some fun and have some laughs" than there was with "Danger: Diabolik".
Star Trek VI: The Undiscovered Country
Again not technically a series finale, although it WAS the final film in the SERIES of films with the original Star Trek cast. And again it seemed like there's was a feeling of "if this is the end for us, let's end it with a bang." I especially like the final shot of the film, of the Enterprise flying towards the star, an obvious allusion to the Western convention of ending with the hero riding off into the sunset, which perfectly reflects Gene Roddenberry's original concept of Star Trek as a "Wagon Train To The Stars".