JAIF

Honestly, what?

There is nothing to argue about.

Aizen vanquished four captain-level people in a row - yes, one of them was only using Shikai and the other was exhausted, but still. He was very clearly implied to have reached the shinigami limits during the SS arc and surpassed them by at least twice that much during the Arrancar arc. His sheer spiritual pressure forced Grimmjow, who was toying around with Bankai Ichigo, to his knees. He commands legions of Arrancar and Hollows. Ulquiorra said that, even after Yammy was trashed, if he fought Yoruichi and Urahara, he'd still have the advantage due to them trying to protect their comrades from harm - Urahara and Yoruichi together. Plus Ulquiorra effortlessly deflected Urahara's shikai attack. Aizen is clearly Ulquiorra's superior. Urahara could, at his theoretical best, equal off to SS arc Aizen, because that's the strongest a 'pure' shinigami can be, but we have tons of evidence to prove that he isn't nearly as strong.