Really though he doesn't need a Bankai. If you don't know the true nature of his power beforehand you're already under it's spell. And even then once you do know you're under it's spell you still can't determine how it's influenced you. It's such a broken power. One of the only powers in fiction that's even more broken is Yhwach's The Almighty which was the ability to know all possible futures and push you towards his desired future and the ability to become immune to any power he understood which the The Almighty would let him see.I don't even think Kubo even knew/knows what Aizen's bankai is