I'd say Itachi. But it's not an easy fight at all. If Itachi manages to hit Bee with Amaterasu and Hit him with the totsuka Blade then its game over. But if bee manages to survive the first onslaught by Itachi. the bee would kill him for sure.
Mmm it depends since itachi hast low stamina all bee has to do is stay away from him and use a TBB, I think bee take this as long as he doesn't get close to susanoo