Its either Zoro or Byakuya. Sauskes amatseru is strong, but Zoro can cut through it (most likely) and Byakuya can overwhelm him with the blade thing. Now the only reason byakuya could beat zoro is if he went straight to flight. imo zoro could counter that with tatsumaki or black thread tatsumaki which keeps going. still flight is his biggest advantage here. zoro could probably tank anything either can dish out at first, but if they jump him theres only so much he can take, but in most likelyhood, they wouldnt.