Yeah, Newhart for sure makes the short list. Just so original and completely unexpected. Of course, it only worked because he did another series just prior to Newhart, and that was still fresh enough in the public consciousness to make the joke work.
MASH was ok, but not really groundbreaking or surprising in any way, just more emotional than anything else. The notable thing about it was the massive viewership. Of course, that was 1983 and most places in the US still didn't have cable. Those that did had 20 or 30 channels so the viewership was not so fragmented as it is now. I doubt you could get those numbers now, and certainly not for network TV series. Closest thing is probably the superbowl.
St. Elsewhere was a very imaginative finale episode, well at least the final scene was. That one actually made a lot of people angry though, as it seemed to reduce the shows entire run to the product of a child's imagination.
Magnum P.I. had a pretty good last episode, albeit a bit incongruous at times.
I warch very little tv now, and the only recent show I really followed was Chuck. First two seasons were excellent. Then it kind of got off track. The finale was good, but not great IMO. I guess I don't care for so many loose ends and unresolved problems. In that regard, it came up short for me. It did have some prize moments, notably Jeffster's cover of "Take On Me".
Not many other series finale eps stand out in my mind. Most have been disappointing to me in some way, and many good shows got cancelled without a proper send-off.