I dont know if its been stated anywhere but is there any chance that Ulqiorra could actually be the strongest espada regardless his number. Just think about it. Ulquiorra is the only espada that has a second release form and i am guessing that since Aizen ranks them and is supposedly unaware of Ulquiorra's second form, then he could actually be the strongest even stronger than Yami. The next thing is that his reitsu is described to be so dense that it felt like an ocean was covering the sky or something like that and it didnt even feel like actual spiritual pressure. I have not seen Yami fight against anyone who was capable of beating him yet, but only time will tell. All i know is that to me as of right now the strongest espada is the one that resembles a devil the most just because of what his transformation symbolizes.