I knew what I was voting for as soon as I saw the title of this thread.
Christmas Eve on Sesame Street-- I used to watch it every year. I don't know if there's much else I can say about it with what everyone else has stated so well. But there's another reason why I will always remember it fondly: it was an introduction to "classic" S.S. for me, being born in the late '80s. The story centered around Big Bird, Kermit was there "on the case", I got to see Olivia (though I also knew Olivia from Follow That Birdl) and for years this was all I knew of David (not to mention, he dated Maria?! What?? LOL
). I don't think I ever thought about Elmo not being there, because Cookie Monster, Grover, Ernie and Bert and Big Bird have always been my favorites (and Kermit of course). It's all I needed for a good S.S. special.
That being said... I was surprised that I liked Elmo Saves Christmas. I thought it was quite clever and really funny in places. It was more "modernized" but it was nice to have a special that Elmo starred in without it being a complete Elmo takeover like lots of things are now. I could have done without a remake of "Keep Christmas With You", but other than that I'd probably rank this one #2. The only other one in the poll I've seen is some parts of A Special S.S. Christmas, but it was kind of one of those things were I saw enough to know I don't really want to finish it.