Maybe I understood things wrong, but the Espada are the best Arrancar Aizen selected.
Most of them, if not all of them, are Aizen's creation.

Vasto Lordes are Arrancars that further evolve into something even more powerful we have not seen yet, Although Ulquiora with his last transformation could have been one of them, or at least very near.

It's not impossible that the hollow form Ichigo had when defeating Ulquiora is also near VL levels.

But the hollow stages were explained long ago in the timeline, so there's no way to know if the author will follow that now. Or if he remembers/cares... He might just do as he pleases when he thinks that's convenient.

So to sum it up, Espadas aren't even the strongest hollow we've seen yet.