I have been watching a *lot* of South Park lately. Started from season 1 and made my way through season 21. I think I've figured out my top 10 favorite episodes, and they're mostly from seasons 6-11 (the golden age imo). 1-4 haven't aged well, season 5 has its moments but is still a bit rough (but gets major points for focusing on Butters and introducing Jimmy), 12-17 have high highs but very low lows, 18-19 are... iffy but not terrible, 20 is embarrassingly bad, and 21 is a smidge better than 18-19.
As a fan since season 1 episode 1, I pretty much agree with your assessment on the seasons and what the golden age was. Season 21 was actually pretty solid when you compare it to recent seasons, but doens't come close when comparing to the golden era.
I think seasons 4 and 5 have more great episodes than any two other seasons combined.
for me, I disagree. Season 4 was still kind of in the early stage feeling of the first few seasons. Season 5 was really good. I think this was when it started getting really good, I mean Scott Tenorman, enough said. But I think the show really climaxed at season 8 for me personally.