Even though we don't know enough about Keto, I'm going for Ventress.
I believe Anakin killed Keto without really much difficulty, while Ventress has demonstrated to be a magnificent fighter, he gave Anakin a really tough battle in both of the occasion they fought, while Anakin quickly took out Keto and then he took out her master, which indicates he wasn't very tired and it was no spectacular battle either.
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Given Ventress's list of adversaries, I'm inclined to agree.
But we can't throw it past Keto just yet. Look at characters like Anoon Bondara, who have absolutely no mention in the movie, yet are incredibly skilled swordsman. The same go for the likes of Sora Bulq and Luminara Unduli.
I honestly don't think anyone can make an accurate call on this thread without more information.
Is there any other source that mentions Keto besides the ROTS game.