Oh, I don't know... Certainly what you said about Jim being the father of all muppets and Frank being his right hand man is true, but more because it was them who started the muppets. Both of them were very good with voices, (I still marvel whenever I hear Miss Piggy!) And yes, even when Steve's 92, he'll still be "the young buck of the bunch," and even though I love him as we all do, he just doesn't seem to have the vocal flexibility of Jerry, if that makes sense.
I was kind of surprised though when you said Richard was second, (I might have said Jim), but on second thought, it probably is Richard.
So in short, they're all good, but I would definately agree that Jerry Nelson might just be called, "the muppetman of a thousand voices"! Heck, at one point while watching MTI, I actually felt sorry fer the guy, he was working so hard!