South Park always had a close relationship with Christmas. It began life as "The Spirit of Christmas," then brought us "Mr. Hankey the Christmas Poo." For the first eight years, the show had a tradition of providing outstanding Christmas episodes that only really stopped when their airing schedule stopped coinciding with late December.
The third, "Mr. Hankey's Christmas Classics," may not be the funniest or best-remembered... it came at a time when the shock of newness that accompanied the show had worn off, and a while before it discovered its gift for topicality. It's basically a hit-and-miss series of musical numbers, of which my favourite is the lounge act between Jesus and Santa. Considering the two had been introduced in their first short as sworn enemies, it's fun to see them kidding each other and trading off numbers.
Then Jesus shoulders Santa out as Santa can't think of a second decent "Santa" carol, and so the jolly fat man bursts in with a rendition of Duran Duran's "Rio," which to me has been an honorary Christmas song ever since.
I like it, because it turns the idea of the "War on Christmas" on its head, with Santa patently unable to overcome the focus on Jesus in the songs and culture on the season (never mind that "Rudolph" is at least implicitly a Santa song and "Jingle Bells" at least contains a sleigh, Jesus is still pretty well represented overall.) The song they end up duetting on, "Let It Snow," is secular, but not commercial, it's just a nice ditty about spending time together (and possibly making sweet love.) The whole context of it makes the songs so much better.
And for completeness, here's South Park's best contribution to the Holiday canon, "Lonely Jew on Christmas."