I'm going with Zannah. Her defense will hold as it did against Harth, though Ventress is sure to give her a hell of a fight. But I feel that in end it's going to be a battle of attrition and Zannah's perfect for that.
Sirak vs Ashoka(fight me Joker, you nurrd!) Is rather close, I'd say that I've never been overly impressed with Sirak, but he's okay and I'll concede that his lightning will present a challenge for Ahsoka, thusly keeping her at a distance. That said her sabers would allow her to block it and her TK is pretty advanced for a Padawan, which will, in my humble opinion, present a problem for Sirak.
This is a close one but I'd give to ash(iwin)oka.
In the end I think that Ahsoka and Ventress would be a challenge for Zannah, but I'll stay roll with Zannah. Ahsoka would be to tired to present a big enough challenge Zannah and Ventress wouldn't likely break through her defenses with strength attacks. So Zannah wins with Sirak as a casualty. Imo