Muster was a great clay courter but he would never be that high. He only won 1 French Open even in a very competitive clay court field, but one without a dominant clay courter like Borg or Nadal. His only great years on clay in general were 1990, 1995, and 1996. He wasnt even really one of the best any other year.
My list would probably be:
1. Borg- atleast right now still is IMO
2. Nadal- will be #1 in future perhaps
3. Rosewall- amazing to wonder how many French Opens he would have won had tennis been Open era back then