User blog comment:Hollow Mario/Aizen vs Yhwach./@comment-24963493-20150110223612

To the death? I think it's safe to assume that, eventually, Yhwach will be defeated, most likely by Ichigo. Granted that Aizen was technically defeated by Ichigo when Ichigo was weaker, and was then trapped by Urahara, Aizen was not killed by Ichigo. From what we know now, Yhwach is not immortal, as powerful as he may be, but Aizen IS immortal. What's more, Aizen was able to affect Yhwach's sense of time just by being around him, even while being imprisoned. So my vote goes towards Aizen, based one the knowledge we have now.

TL;DR: Based on what we know now, Yhwach may be more powerful and more strategic than Aizen, but Aizen has been shown to be powerful enough to alter Yhwach's senses while being imprisoned just by being near him, and he is immortal, so I think that Aizen would win in a death battle.