Actors Rob McElhenney, Glenn Howerton, Kaitlin Olson, Danny DeVito and Charlie Day act during a dance scene on the set of “It’s Always Sunny In Philadelphia.” The most recent season of “South Park,” which wove presidential politics, cyber trolling and space exploration into a timely tapestry of tastelessness, offered a TV rarity: an irreverent show that unspooled its best outing nearly 20 years in.