The Great Santa Claus Switch (1970)
It's obviously early work, but solid. The plot revolves around Cosmo Scam (an evil wizard) and his plan to replace Santa Claus. Both Cosmo and Santa are played by Art Carney, everyone else is a Muppet.
Sesame Street would have been just in its second year, and this feels very much in tone with Sesame Street. There's nothing too scary for kids (all of Cosmo's monster lackeys have soft hearts) and there's moments of sly humor on the side for adults. Also, plenty of very bad puns.
The music is sweet and funny, and overall I found the hour charming, although it was slow here and there. Later work would learn from this, tighten the pace, reuse puppets and ideas created here. It was an important step in the development of the Henson company.
I found a poor quality copy on YouTube, with fuzzy sound and gaps in the video. Unfortunately, I think this is probably the only way to see The Great Santa Claus Switch right now, besides seeking out one of the museums that have a copy. As much as I'd like to recommend this special, the quality issues mean that really you should probably only seek it out if you're a major Muppet fan. If the rights holders were to restore it and release it on DVD, it would be a very nice holiday special for families.