Well, obviously I can't choose. I'll have to do both.
Here comes the spoileriest blog post ever. I bet no one even clicks on this link for fear I'll ruin a show they haven't seen; so to mitigate that loss of traffic, I'll do two top fives.
(Funny, most series fall somewhere in the middle, and even some of those that are considered 'best' by some are considered 'worst' by others. Take Seinfeld and Roseanne as example.
Top Five Worst TV Series Endings
5) Lost. Fuck lost.
4) Roseanne. Turns out, they didn't win the lottery after all. In fact, the series is a novel being written by Roseanne, and John Goodman's character has been dead for years. Ouch. (This is one of those ones that's creative enough to be almost good.)
3) Quantum Leap. "Dr. Sam Beckett never returned home."
2) Dinosaurs. Yea, this show ends with the daddy dinosaur explaining that they're all boned, ice-age style. Strange choice, family show.
1) Dexter. A great show that was arguably two or three seasons too long. Not so bad as arguably being one episode too long. This was a shit.
Top Five Best TV Series Endings
5) Seinfeld. They end up in jail, just bickering at each other as though nothing has changed. Some hated this. I thought it was perfect.
4) Freaks and Geeks. I loved this show, and fell for all the characters. Watching them move on with their lives was tough.
3) Cheers. Nothing special, just Sam choosing the bar, and things go on without us.
2) Six Feet Under. Odd to find out how everybody eventually dies at the end of a series. This show was so good it would've been hard to fuck up the ending; instead they went the other way.
1) Twin Peaks. Iconic for a reason. In the end, our hero is beaten.
Short Answer: Honorable Mention goes to MASH, a show everyone loves and considers great, but also a show I could never get into.