@narutobleachfan: Yamamoto was always effected by the KS, just like Yhwach, but they both found a way around it. You're right that Ichibe was supposed to be around head captain's level, being especially OP compared to everyone else and giving Yhwach a hard time. Still doesn't put Aizen on their tier.
Think of it like this. Yamamoto is Highfather, Yhwach is Darkseid, and Ichibe is Odin, with Ichigo being Superman.
@narutobleachfan: Mimihagi absorbed Yhwach is basically an amped version of regular Yhwach, so yeah he could defeat Yamamoto.
As for the scaling, Aizen is not greater than the head captain, which he admitted. The head captain is so far above everyone else that, even when Aizen was at his strongest fighting Dangai Ichigo, saying either of them is actually above Yamamoto is unsubstantiated. And as we saw in the final arc, the gap between Yama/Yhwach and everyone else is huge, since those two are on similar levels (before Yhwach got all his amps). So Yamamoto >>>>>>>>> Aizen
@narutobleachfan: You are very much mistaken. Old man Yama got oneshotted when his bankai was depleted on Yhwach's meat-shield, and when his back was turned. Only at his absolute weakest was Yamamoto defeated by Yhwach, but Yhwach already admitted that Yama could easily vaporize him and everyone else, and would defeat him otherwise. Yamamoto>Yhwach, it's canon.
Yama has gone soft, not weak, which is the only reason he's not a war potential
That didn't save him when he's fighting Yhwach, except once only when the plot forced him to inexplicably defeat an insurmountable enemy via handwaving. Yamamoto can take on Yhwach 1on1 and nuke everything else in the process. Because he's simply more powerful, and without plot devices you can't get around him (Aizen did his research, I guess Yhwach stole his idea).
XLR87T3's comments