Most of my favorites have already been mentioned, many several times. Thanks to all the digital TV channels I'm able to watch many of my favorite old shows. Among my absolute favorites:
Barney Miller
Newhart
Night Court
Crime Story
Magnum PI
Homicide
Numbers
Zane Grey Theatre
Laramie
SCTV
SCTV was way better than SNL. A tremendous cast and great writing created some very funny TV. Their parody of "The Godfather" was priceless, as was "Ben Hur". Probably my favorite show.
There were also a lot of great shows from across the pond. Anything that ran on Mystery, especially when Vincent Price was host, was excellent. The Britcoms are classics also. Some of my favorites from our British friends:
Are You Being Served
Sherlock Holmes (with Jeremy Brett)
Miss Marple (with Joan Hickson)
Reilly: Ace of Spies
Father Ted
Last of the Summer Wine