Board Thread:General Discussion/@comment-24382631-20140110234616/@comment-8986886-20140111013112

According to Yamamoto, his Zanka no Tachi East would destroy anything it touched, leaving no trace. If he were to land a direct hit on Aizen, then he might have been able to destroy the Hogyoku, before it had a chance to revive him; both Gin and Ichigo were able to critically wound Aizen's Hogyoku forms. Yamamoto's Zanka no Tachi West also shielded him against direct attacks, by disintegrating anything it came in contact with, making it extremely difficult for anyone to kill him.

Yamamoto is acknowledged as having been the most powerful Shinigami for the past millennia, as well as having had the most powerful Zanpakutou. In fake Karakura Town, Aizen had to seal the flames of Ryujin Jakka and use its power against Yamamoto, to defeat him. Similar to that event, Yhwach also needed to steal Zanka no Tachi from Yamamoto, before he could kill him. In fact, I believe that to be the only reason why the Medallions were made in the first place. The other captains were merely test subjects, to make sure Yhwach's plan would succeed.

So, yeah, he probably could have defeated Aizen.