Just a thought, because i realized that since Ulquiorra never showed his Segunda Etapa form - does this make him the True Primera Espada? (I know his has probably been discussed before)
This is just a speculation obviously but lets look at what happened with Ichigo's power scaling. When he fought ulquiorra, he was unable to wound him significantly and only managed a a very shallow cut on his chest. Then when hollowfied, he was able to pressure him but could not even harm him in his resureccsion with a full powered, hollow masked enhanced getsuga tensho.
Then after the fight, he goes into the lower level of Las Noches and was able to cut Yammy with a getsuga tensho, even after he was exhausted and was low on reiatsu (Unohana comments on his coat being a gauge to how much reiatsu he had left and the more damaged it is, the lower it is) and he said his hollow mask felt different. And technically, Yammy as the 0 Espada should be 3 levels above Ulquiorra, so at Ichigo's current level, his attack should've tickled him.
What are your thoughts? i truly believe Ulquiorra was the strongest of the Espada