As I've said many times before, Marty really made Telly his own and really brought so much life to the character that was lacking in Brian's day; not that Brian wasn't good or anything, he was really one of the more versatile performers they had, but Brian's Telly was a really wimpy character - always mopey, always pouty - he was more of a Muppet Eeyore than a Muppet Woody Allen. Same with Snuffy, Marty gave Snuffy a larger range of emotion that brought more life to the character, as opposed to the perpetual, "Ohhh deeaarr. . . . hiiiii Biiirrrd, I'm sooooo saaaad. . . ."
Ruby had potential to be an interesting character, especially at a time when female Muppets was a big concern for the show . . . shame she didn't last, though it could be she was such a tomboy that maybe she kind of defeated the purpose of having a female Muppet little girls could relate to.