Man, been a while since I'm on a versus thread, oh well.
Yamamoto should scale above Aizen since he admitted Yamamoto is more powerful than himself (Hence why he decide to make Wonderweiss able to seal his shikai). And Aizen is waaaaay above Toshiro in his bankai form (
657 megatons) where he
casually oneshotted him and many other captains and vizards during arrancar arc (He also oneshotted Toshiro and Komamura during soul society arc). So he actually has the AP advantage since he > Aizen >>>> Toshiro = 657 megatons.
He also stomps and one punch
Wonderweiss who is comparable to vizards like Kensei and Mashiro. He punched Wonderweiss so hard that his Regenerationn couldn't save his body from being
destroyed. And he's not even use kido, zanpakuto or anything. Just pure raw power of his technique.
Speed gap is very small, yamamoto being 1058 mach vs Natsu's 1381 mach. He also superior to Kyouraku in speed as we saw this during soul society arc, not to mention he got speed amps aka shunpo while Natsu does not. He can stand on the sky for better mobility.
And shunpo and flight is useful against aoe attack like Ichigo used shunpo to avoid senbonzakura and against danmaku like
this.
He also has experience advantage since he's at least 2000 years old as commander of Gotei 13, a master of zanjutsu and hakuda, a feared, respected and even stated as the strongest shinigami for
thousand of years that both Aizen and Yhwach (and also defeated him 1000 years ago) need to resort to use some kind of strategy instead facing him directly. A mentor to veteran captains like Shunsui and Kyouraku, so he should be more than smart enough to not rely on his fire when he know Natsu has resistance to fire and able to absorbs it. And this is not first time he had to rely on Zanjutsu and Hakuda, so he has experience and skill advantage here.
He also able to use forbiden kido #96, and
he used it on Aize while weakened. And Aizen is definitely NOT a fodder.
And he likely has stamina advantage too via scalling to Ichigo (who trains in dangai for almost 3 months while his father Isshin
continously used his reiatsu to stabilize the dangai, or when Ichigo trains for 5 days with Urahara in the earliest arc (And that's a few examples on how insane shinigami's stamina and endurance is, other shinigami also has many endurance feats like when Byakuya is still alive when most of his body is nearly destroyed by senbonzakura). So yeah most shinigami can take a lot of punishment as even when he
got cut in half by Yhwach, he still able to briefly
grabs his cloak, and as I said before, even when weakened he still able to use forbidden kido (a high ranked one too).
He also has extrasensory perception to help him against surprise attack or to located Natsu, a small advantage I know, but still an advantage regardless.
So I think Yamamoto can win via AP, mobility via shunpo and flight, skill, stamina and experience.
