I’m going to jump into the future a couple years and predict Better Call Saul. The trajectory of that show is just unstoppable.
My current favorite series finale is a tie between Justified and Halt and Catch Fire. Both were subtle, melancholy, and gave satisfying closure to all three leads.
Other greats have already been mentioned here multiple times: The Wire, Breaking Bad, MASH, The Shield, STtNG. I was just a couple years too young to understand Newhart’s ending, but I can still remember my dad freaking out when it aired.