Season One was them getting their feet steady. I think that Seasons Two through Thirteen were consistently very strong (Thirteen ended on one of my favourites, Poppa's Got A Brand New Badge
). After that, it started to decline, and I stopped watching it regularly (but then, I slowly started falling off TV altogether then).
I occasionally tune in, and they can still produce some really fun stuff, but it's not must-see anymore. But whatever, still the greatest show in TV history.