It's the fact that she has defeated Jedi Masters in combat before that makes her able to beat Bastila. Her fights with Plo Koon and Luminara were fights that she couldn't win, but she was able to do some damage. I doubt Bastila would have the same luck.
And yes, Bastila is powerful. But Ventress is in a whole different league.
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....