An'ya Kuro really fascinates me. It should be noted that before Vader confronted her, she had been in solitude and had not seen combat in over 16 years and was also fatally old. Despite all of this, her reflexes and speed are astounding, and her skill with a lightsaber is great enough to match Vader for an extended period. Imagine how formidable she was during the Clone Wars!
That said, for this match, I think I'll give it to the Dark Woman. She was not only very strong in the Living Force, but also housed several unique powers such as invisibility, teleportation, plant control, and she could even phase through solid matter. Considering she's the lady who produced Jedi legends like Ki-Adi Mundi, Darth Krayt, Aurra Sing, and Jon Antilles, I wouldn't put it past her to be on Ventress's tier herself.