Yeah, in most minor spoiler way possible for this season, [spoiler]the mid-season finale and return basically set up a drastic change to the status quo of the show and then killed it at the last minute and never mentioned it again for the rest of the season. It was disappointing mostly because the unexpectedness and the way Matt and Trey have been on record saying that their hearts and creativity have been on Broadway made it seem like they might actually try to make some changes in order to reinvigorate themselves. Alas.[/spoiler]
Of the last three seasons, I'd probably recommend (in order of airing) The Ring and The Coon from season 13, Medicinal Fried Chicken (if only for the goddamn music making me laugh like a lunatic at certain bits) the two-parter (200 and 201) and the three-parter (Coon 2 etc) from season 14, and You're Getting Old, Ass Burgers, and Broadway Bro-down from season 15 if anyone was going to cherry pick. Even the weakest episodes have generally had something amusing about them, though I honestly couldn't tell you why I've actually kept up with South Park after dropping both Simpsons and Family Guy, since they all seem to have about the same hit or miss ratio these days.