Frank's Kermit and Greg's Fozzie were the closest, I think. They even used similar voices for their adult counterparts. Personally, watching the Little Muppet Monsters clips in "Six Flags over the Nursery" the one voice that is terribly off is Hal Rayle's Miss Piggy. I give him credit for trying, but it sounds terribly wrong.
I think Russi Taylor did a smashing job as Baby Gonzo as well. She really had the vocal patterns of the character down pat. In an impersonation, it's not just mimicking the sound of someone's voice, but it's also mimicking the speach patterns.