I think the best had to have been Newhart. All other shows wished they could pull off such a great ending.
I also liked the finales of MASH (my favorite show of all time), St. Elsewhere and ST:Voyager
The worst has to be Lost - I was very into Lost for a first three seasons, but then "lost" interest. Last year I watched the rest of the show on Netflix and found the ending a complete rip-off. In fact, the last season, if not more, was crap.
Seinfeld, Friends, Quantum Leap and Cheers also are near the top of my "worst finale" list.
The ST:Next Generation finale was OK, but, since I HATED every Q episode, it was a little spoiled for me from the start.
MotionMan