Zaraki stomps Yamamoto badly
Unohana can canonically tire out Aizen. This isnt a "Aizen just thinks" situation as Kubo wouldnt answer this way if that werent the case. Additionally, one can reverse the argument and say aizen thinks he will win but loses because he only thinks he will win not that he would actually win.
Aizen and Yamamoto are the pinnacle of shinigami during FKT. Aizen only says Yamamoto's raw power is superior so he would lose in a head on fight, but Aizen's shikai is anything but head on.
Saying "Aizen used wonderweiss" is a poor argument as the geezer too would rather hide and set up a technique to commit mass suicide than fight Aizen 1v9 with Shunsui, Toshiro, Komamura, Soifon, Shinji, Rose, Love, and Lisa. Yamamoto in fact chooses to use EJ instead of RJ even after grabbing the real Aizen.
So its not Aizen alone who was using a cheap way to stop Yamamoto, as Yamamoto was going to use a cheap way to stop Aizen. Its a war, neither side cares about winning a fair fight as shown.
So Unohana who can canonically tire out peak Shinigami pre-TYBW is narratively surpassed by Shikai zaraki. Zaraki in base alone is somewhere in between Base and Bankai Unohana as his kid self was above pre-kaido Unohana but zaraki himself is an adult now, and his shikai should narratively surpasses Bankai Unohana.
That would mean Shikai Zaraki alone is at the same level as Shikai Aizen and Bankai Yamamoto since he is just above Unohana who is vaguely under those 2.
Then he gets a massive boost with Bankai that would easily brute force through anything the geezer tries to do. Yama would get blitzed and have his body alongside his sword cleaved in half by a single swing.