Dear Avira,
The qualification that the poster gave was gaon. Not depth or analytical strength. Gaon means vast knowledge. The Rebbe was unique in his vast knowledge.
There are many living people who heard his Torah. There are also a select few geniuses that studied with him. He astounded everyone. One Gaon Rosh Yeshiva of today told me that his learning level was on par with Rav Shach. Also, the bnei yeshiva of today considered him the biggest lamdan among all the Rebbes. They went to his farbreingin.