I actually kind of like seasons 2, 20, and the first half of season 16. I honestly have trouble differentiating between season 22 and 23 off the top of my head, but I'll just go with 23.

I'd say 23 because it had way too much Tegridy Farms. 12 and 22 were okay and I really like 16.

It comes down to 20 and 23 for me, and I'm having a hard time picking between them. They're both bad, but for different reasons.

20 is more incompetent, mainly due to switching gears midway through, abandoning plot points, overusing jokes, and the serialized nature means none of the episodes can stand on their own.

23 shoves Randy in our faces, taking away from actually interesting plotlines (like the ICE detention center) because we keep cutting back to Randy acting like a douche. There's also just a spiteful nature to it, with the creators practically joking about how the viewers don't like the Tegridy Farms plotline, but continuing to use it anyways.

I'm more forgiving of Season 2 because it was early and they were still starting out. 12, 16, and 22 were just average, nothing too terrible.

EDIT: Went with 20 for now, because 23 at least has standalone episodes that can be enjoyed on their own, while 20, as a serialized storyline, means the episodes can't stand on their own.

I chose season 20 but I was pretty torn between that season and season 23. I have mixed feelings on season 20 because of the rushed and undercooked storylines, derailing the characters, outdated Trump humor, the Cartman and Heidi relationship was extremely forced and uninteresting, that Danish troll-trace guy was a very lame villain, not helped on how the twist was really stupid and felt unnatural, and it was annoying how every episode ended on a cliffhanger and therefore none of them were watchable out of order.

And season 23 kept shoving Randy (Matt and Trey's ever-so-beloved creator's pet) in our faces, and kept trolling us with the Tegridy Farms storyline, which got really old only after a couple episodes. I really hate how they seem to be well-aware of all the criticism that storyline got and yet they still continued to have it hog all the spotlight, stealing away the screen time of much more interesting storylines.

Who said season 2 was bad