You're Getting Old Is Fifteen Years Old
https://www.cracked.com/article_41321_thirteen-years-later-south-parks-youre-getting-old-still-hits-too-close-to-home.html
I'm not sure if this is the best South Park episode (although it certainly must be up there) but I think it perhaps, literally, represents the show hitting full maturity.
It had already moved on from the early season base humour but this was dark, dark stuff. And definitely an episode you remember long after the others have kind of blended into the same thing or the later day specials we have now...