I have to agree that this is probably my favorite version. And, having just read the book this year, it pretty well captured the feel. The book does bounce around from dark humor, horror, and joyousness, that I think The Muppets are uniquely able to capture. Muppets can be so many things, but you never have a trouble doubting that a crooked spider, a talking pig, and a, whatever the hell Beaker is, could all be inhabiting the same space in the same universe.
Personally, I think my favorite Tiny Tim, or at least, the one that gets me the most emotional, is from Scrooged. Like you said, it's difficult to capture the proper level of pathos and childlike innocence with a real child actor, but I feel like Robin might still be a bit too cutesy. What I liked in Scrooged was that his issues were more cerebral. I find it far more believable that Scrooge's turn (or Cross in that case), can help Calvin (?) overcome his struggles far more easily than it will cure Tiny Tim of whatever ailment he was afflicted with. Plus, having a mute child's first words be "God bless us, everyone" (and Alfre Woodard's reaction) makes that moment that much more powerful.
Growing up, I think my favorite version was Mickey's Christmas Carol. I still like it a lot, but I think it's only about 24 minutes or something so it doesn't quite have the weight of a full length movie.