Kizaru.
1. Kizaru has never gone all out: despite his more active role in the War at the Summit.
2. Kizaru has range over Doflamingo.
3. Kizaru has speed over Doflamingo.
4. Kizaru has logia state (which, based on the relatively comparable Admiral levels), has some resistance to cutting attacks despite another's use of Haki.
Realistically speaking, a serious kill intent Kizaru has no real reason to lose until some limitation to his DF is revealed. So until then...
Admiral "Yellow Monkey" Borsalino.