Realistically he goes below Ay (Ay > Gyuki = A3). But since we're doing a bit of WA powercliffing, A3 ~ Gyuki is mostly an outlier and we don't even get to know what exactly brings Gyuki to his level.
Therefore, I can rank him the highest plausible, which is actually right below Gengetsu.
Gaara and Gengetsu talk about how you must take down the strongest enemy first, and Gaara says they did by bringing down Mū. Then Gaara considers it and says Gengetsu could have been in the conversation. The implication is clear: those two are above A3.
This is consistent with A3 only being ~ V1 Ay in speed, Mū outreacting him, and in multiple translations the implication of Gaara actually having the better shield (Some say it's only after A3 is dead, don't ask me which one is correct). Even if for some reason you ignore Mū being narratively stronger and having an arsenal that would win him the fight 1v1, A3 is regardless still stated weaker than the white haired crippled Edo Nagato who was being overwhelmed by V2 Killer Bee (Ranked right above Mū) until the Chakra was absorbed, so at best he can get right above Mū without making the tierlist inconsistent. I'd still disagree, but it's much better than putting him in High Kage for beating nobodies.
