The most important thing about her bankai is that she didn't need to hit Zaraki with the blade to destroy his flesh. She can win if her reiatsu is strong enough to get past his zanka no tachi west.
Another important factor of this battle is zanka no tachi south. From what I can tell, Unohana's bankai only affects flesh and not bone. She might not be able to disinigrate them, but I also believe that should would be able to avoid being captured by them, and destroy them some other way, or just bypass them and go stright for Genryusai.
I voted for Unohana, because she was given the title of strongest shimigami while Genryusai was her leader, she said she was the strongest while knowing what he was capable of, and I have to believe that she is strong enough for her powers to reach him, which he cannot heal from on his own.
It would be extremely tough either way, but I think she is one of the few people who could do it.
