The Simpsons (Season 4 was the pinnacle, everything else was downhill)
MASH (They did everything they could do, and still went for three more years)
Cheers (All the characters became cartoonish)
Boston Public (First two seasons were outstanding, the last two were too soapy)
Who's The Boss? (When I was 12-14, the images of Alyssa on my TV screen assured me that I was indeed attracted to hot brunettes. But the show got really stupid really quickly)