Homicide: Life on the Street
Babylon 5
NYPD Blue
Barney Miller
All in the Family
Millenium (then got worse after Season 2 again)
Taxi
Simpsons
Family Guy
King of the Hill
STS9
ST: Voyager
I always think of the life of TV shows like the life of a person. The first few years are spent finding out how to walk, "getting your legs", so to speak, finding out what works. The second portion is the glory portion; ;hitting your stride, having formed a real identity. The last portion gets weaker again as things begin to wind down and forward motion slows a lot. It's pretty much like that for most (though not all) shows.