I always saw him in the Commander range even if on the lower parts. Attacks like Dai Enkai Entei are pretty crazy and take care of pretty much anyone weaker than Commanders.
His feats in his novel and the manga adaptation were pretty crazy but I understand if we're excluding that. His fights against Jimbei and cuffed-Yamato are pretty good feats, especially when that was Ace a few years before pre-timeskip. I don't think any rando with Ace's fruit would be cancelling out Aokiji as efficiently as Ace did so while his fruit is a good counter, it's still a feat for Ace to counter an Admiral's attack. Busting through a number of Baroque works ships and tossing Blackbeard's crew around (plus Teach made clear Ace was too much for the rest) with one hit's also pretty good. Plus, him being invited to be one of the Shinchibukai also highlights his strength, especially when he hadn't been a pirate for that long.
Yeah, Ace's physicals and haki feats could've been better in the manga but his neck didn't break from Teach's chop, he said it almost did but later on, we're literally told that for a dude who'd be expected to be super devil fruit reliant, he was putting up a great fight even with it being countered by the yami yami no mi and that he truly does represent a formidable opponent. His final attack against Teach's in my opinion kinda highlights he's in the Commander range, I don't see many characters busting out such and it's not like it came with apparent drawbacks, it was just a mini-sun he uses as a finisher that spanned nearly half an island.
His potential was also pretty stellar. Conqueror's haki, an amazing fruit, the son of the Pirate King who was taken in by Whitebeard. He'd have made it far and would've reached top-tier in my opinion. As is, I have him as one of the strongest YC4 (possibly weakest YC3 but basically around that area) if you care for those terms. I don't see him near guys like Marco though, that's too much.
