mik5,
I don’t think I am in position to say whether or not Rabbi Zevin was a bigger talmid chacham or tzadik than Rav Shach. (The relevance of being a bigger talmid chacham is because one of the factors in being machria among opinions is gadol b’chochma).
Obviously, in many cases the issue is clear (next-door neighbor compared to Reb Moshe), but when you are dealing among world-class talmidei chachamim and tzadikim it isn’t clear at all. Can you judge who was greater posek between Reb Moshe and the Tzitz Eliezer? Who was a bigger talmud chacham between Rav Zevin and Rav Schach? Who was a bigger tzaddik, the Chofetz Chaim or Rabbi Aryeh Levine? I can’t.