i'm gonna ramble A LOT but in my opinion, seasons 1-4 are the "classics". i feel like cartman changed after the episode 'scott tenorman must die'. seasons 5-9 was great, had some really good episodes but a couple forgettable ones... still judging by seasons, 10-17 had a lot of great episodes; i am probably biased since i grew up with these seasons. i feel like season 18-19 is okay but this is where the #cancelsouthpark thing was brewing. season 20 onwards felt reaaaally different, the episodes linked to each other more. the last 4 seasons however have been pretty good, and while a lot of people despise randy's 'tegridy farms' i actually like how much of a jerk he is. i wonder when they'll drop tegridy farms.
i can't really pinpoint a "golden age" but i think the episodes during the 1998-2011 were pretty great, maybe it's because cable was still widely used, and the internet was emerging to the public.