Ti.
I've argued this before. Kenobi's above her as a duelist, but mainly as a defensive duelist. His offensive capabilities shouldn't eclipse hers. His style revolves around playing the defensive and waiting for his opponent to slip up, but that would require him surviving against her Force Powers up until a time where she does slip up. I'm not even confident she would, she was a Jedi whose ability to manipulate battles was as famous as Windu's skill, Yoda's wisdom or Mundi's courage. She'd know Kenobi was waiting for a mistake, and wouldn't give him the opportunity.
Don't mistake me, I don't believe Shaak Ti is a greater battlefield tactician or duelist than Kenobi, but form for form she comes out stronger, and her skill set almost seems designed to exploit the flaws in his...