Since I watch mostly classic tv shows a lot of what I consider to be 'best' may differ from others. In no particular order: The Love Boat Happy Days Laverne and Shirley Brady Bunch Partridge Family and many more
the Brady Bunch, Bewitched, & I Love Lucy come to mind. The one from Three's Company used to bug me because I still can't figure out the lyrics... Oh, & the Flintstones, and the Jetsons! and ...
surprised no one has named TWILIGHT ZONE theme. maybe no one else is old enough to remember it, but 60-year old reruns are on the SYFY channel almost every day. it remains one of the most iconic themes ever for a tv show. it is one of the very few tv themes that is recognizable from hearing just the first 4 notes played.