Early on in the war, she backpedaled and jumped off a cliff to get away from Windu. She's easily one of the top ten in the era as of Obsession, but Vader could probably crush all but three or four of the PT greats using the Force alone.
That said, I can definitely see Ventress winning the duel. I'm leaning towards her, in fact. But she has absolutely no chance in an all-out scenario.
She has superb skill in sabers, and agility that outdoes Vader by a long shot. But his TK is DAMN.
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A little off topic, but do you believe, then, that Ventress could defeat General Grievous or Kit Fisto?
To quote Labyrinth of Evil and the Complete Star Wars Encyclopedia, Grievous surpassed all of Dooku's disciples in swordsmanship in "weeks." And he did defeat her and Durge together.
She's already defeated the latter, and within the first year of the war at that. Grievous? Eh. It's conceivable, but only because TCW has knocked him down four or five notches.
Again, at the beginning of the war, and also with the necessary advantages of fear, surprise, and intimidation. Grievous is capable of taking down just about anyone if he starts the battle with those on his side, but without them he isn't quite as dangerous; look at the nature of the battles he's won versus that of those he's lost.
For Kenobi and Ventress, the fact that Kenobi is the master of soresu probably had a huge impact on the duel.
As for Ventress and Kenobi... What about it? He might've never defeated Ventress, but, as far as I know, Ventress never defeated him, either. Although, in all honesty, I don't know the occasions in which Kenobi and Ventress fought, apart from the CW movie, so I can't really argue much about that.
The passage from LoE represents the beliefs of Grievous himself; it's from his perspective. I've never seen the one from the Enyclopedia.
And assuming Grievous is the superior technical swordsman - the LoE passage makes specific note of Dooku training him in "lightsaber technique - then Ventress's defeat of Fisto needs to be explained. Granted, she had observed his unarmed fighting style for several minutes prior to dueling him, but Grievous had already gone against up Fisto once, and he had already been trained in Shii-Cho by Dooku. There's no mention of Ventress using the Force against Kit, either, and I have immense trouble believing that she has a more unorthdox combat style than the four-armed cyborg who can rotate his wrists and fight with his feet.
I have no idea when the CWC movie takes place, but The Cestus Deception is set a year into the war, and she completely outclasses him then. She has him on his back in literally two exchanges, and he only survives because the heavily wounded Fisto cuts the supports for the dock they're on and drops them both into the water.
Yeah, Ventress is good. Still, she was little more than a tool for Dooku and Palpatine, who describes her as an 'occasionally capable warrior. Nothing more or less.'