Hoo boy. This is a tough one. Normally I would say Deneve all the way if there wasn't for one thing. Helen's fight with Isley. During that fight it was very clear what kind of potential Helen has. All she needs to do is get serious. Her arm stretching and drill sword abilities can work together and are very dangerous when they combine (if Helen managed to hit just a bit higher she could have killed Isley).
Deneve doesn't have such luck. Her abilities dual wielding and regeneration don't work so well together and are more useful against opponents like Duph.
I have to think about this a bit more.