From what I saw on the Cartoon - Ventress was beating Luminara (imo a fight Ventress definately could win) and it was Asohka Tano that saved Luminara....(never saw the Plo Koon one - but I can Imagine Plo Koons Physical strength and form V giving Ventress some problems)
And didn't Ventress also beat Kit Fisto in some comic?
So Yep - another vote for Ventress from me - Ventress does have her weaknesses but nothing that I can see Bastilla would be able to exploit....
I think the Plo Koon one was in a comic. Anyway. Ventress has beaten Jedi Masters in battle. I doubt Bastila could do the same.
It's been awhile since I watched the Luminara fight, but Ventress was winning.