I feel like the numbers on their back aren't really good indicators. For example, Ulquiorra said he hasn't shown his second release form to anyone. It could that suggest he's the strongest espada.

Here's my opinion on the ranks of the Espada's (Assuming at full power):
1. Ulquiorra
2. Baraggan
3. Coyote
3a. Nel (adult form)
4. Noittra
5.. Harribel
6. Grimmjow
7. Yammy
8. Rureux
9. Aporro
10. Arruruerie

The only adjustments I've made to the original list are Ulquiorra, Coyote, Noittra, and Yammy. I believe Ulquiorra should be the strongest Espada since he's the only one that achieved second release form. In that form, his agility and strength seems to be far superior compared to the higher number espadas. I moved Coyote down to number 3, because how shunsui dodged his cero with ease kind of indicates that he is a bit lackluster. I moved Noittra up a number, cause Ichigo couldn't even scratch him in his bankai form; he was also a formidable opponent to Kenpachi. Lastly, I put Yammy as no.7, because it kind of disturbs me for someone to be the most powerful espada who possesses only brute strength.

Anyways, this is purely my opinion with some logical reasoning. Please share your order/thoughts of Espada's ranks, and support it with good reasoning!