Shinigami Aizen vs Yamamoto is a 50/50 fight, anything else doesn't really make much sense
Everyone is always bringing up Aizen creating Wonderweiss specifically to counter Yamamoto and his statement about Yamamoto's power, like Yamamoto didn't literally spend most of the battle setting up a suicide nuke move, using other Captains as distractions, that was guaranteed to kill not only him, but also all the Soul Reapers present in FKT, as well as produce enough flames to annihilate the entirety of FKT along with the barriers, just to take out base Shinigami Aizen. You can't even use the argument that he was holding back his Bankai because of its destructive nature, because the option he chose was somehow even more destructive, required preparation and, unlike his Bankai, would've 100% resulted in death of everyone involved. I just can't fathom why he would choose to do that if he thought he had a good enough chance of beating Aizen in a normal fight, and he's usually extremely confident in his abilities.
And look, I'm not saying that Shikai Aizen is equal to Bankai Yamamoto. But most people are forgetting that Aizen has a Bankai as well, which Yama is most certainly aware of, and, regardless of what you think it does, we're yet to see a Bankai that doesn't significantly increase one's fighting power compared to their Shikai. My take on this is that both fighters had so much respect for the other's abilities that they didn't want to risk a direct battle, and therefore both took precautions before the fight. Aizen didn't think it was worth it engaging in the only fight he had a chance of losing, and Yamamoto evidently didn't want to gamble the existence of Soul Society and the Soul King on his ability to defeat Aizen in a 1v1 fight. That should be a good enough evidence to scale them as equals or at least on the same level, which is also in line with the narrative of being the peak of Shinigami that both of them have.