no he's ranked espada nr.4 above him are:
espada nr.3 - Tia Harribel.
espada nr.2 - Baraggan Luisenbarn.
espada nr.1 - Coyote Starrk.
espada nr.0(/10) - Yammy Riyalgo.
Well, i would have to agree because ulquiorra practicality would be the only espada to survive for a while because put it this way he was the only espada to form a second release which proves his hidden power and strength. :)
No, we have no idea who is the most powerful Espada. Going by ranks, Yammy would be the most powerful. However, with Ulquiorra's second release stage, it skews the power rankings making it impossible for us to truly say who is the strongest.