Well we all know Hitsuguya will get owned in a matter of seconds by Halibel since he pretty much got destroyed by Luppi(former 6th espada who seemed to be a hell of a lot weaker than Grimmjow), but I wouldn't count out Shunsui or especially Yammamoto since we have no idea what they're capable of.
but they also set up a thing at least in the series where it made it seem like hitsuguya could wield his bankai but hasnt mastered it.
dont know if they potrayed the same message in the manga.
And like ive said in the series thread. They have decided to give the captains superman strength styling.
Superman on occasion has trouble picking up a car, other times hes able to stop a plane crashing at full speed or simply fling cars around with ease.
If memory serves they stated that more or less the
Gillian - took a squad of newbs to take down
Adjuchas - dont remember what was said about them
Vasto Lordes - stronger then a single captain
now making the vasto lorde class into espada they became MUCH stronger.
So unless the captain levels who were for the most part embarassed by the newly functional bankai and untamed vaizard capabilities of ichigo.
They basically make it seem as though the captains were much weaker then ichigo.
Captain commander i dont know much about. And a few other captains have had limited combat display.
AFAIK we also have limited knowledge of yoirichis full capabilities with her bankai and anything else, and uraharas.
Likewise we dont have much about any of the vaizard who are apparently all old high ranking members in the protection squads