I would say Firefly, had they not gotten to wrap things up in a movie. OMG, I lament the loss of that show still today.

Conversely, one of my favorite endings of a series hands down is Newhart. Maybe you remember Daryl and his other brothers Daryl? It ended with Bob waking up with the actress who played his wife in the first series, and so Newhart was just a dream! I wish more shows were this thoughtful.

Another thing I would like to see, when television does a flashback episode, which invariably become the 'jump the shark' episode, they should have pre-recorded footage from the vantage point of the other actors and actresses- or from POV of the person dreaming/remembering, so it's not done by just reusing existing footage.