Dear Avira,
At one point, many rebbes were convinced that The Rebbe was about to reveal himself as moshiach. Though they moved away from him while he was still at the peak of his abilities.
This still reverbarates behind the scenes, and has an effect on many issues in chareidi society.
I don’t know who brought about more learning, it’s very debatable.
Though it would seem that Rav Shach was the greater hogeh batorah.