The first season.
I read Trey and Matt hate the early episodes but I am rewatching Season 1 and it is still laugh out loud funny as sh,t.
www.thecultofhorror.blogspot.com
The first seasons of any show are usually the best, it's after that that series go downhill. South Park is still going strong.
shareSeason 1 is usually one of the weaker ones though, because everyone is still trying to figure out the format, the pacing, the storylines, how to develop the characters... etc.
Usually by Seasons 3 and 4 the shows are at their strongest, and sometimes it goes a bit beyond that.
I agree that SP is still pretty good overall, but there was definitely a downward slope in those mid-teen seasons.