i think it would be yamamoto genryussai and zaraki kenpachi they both have different battle style and whoa! they're really really F-ing strong specially yama-ji

For straight power, as in without any additional abilities, it's obviously Kenpachi. For the use of shikai, could go to either Yama-jii or Aizen. For what we've seen of bankai's, Ichigo, the Vizards or Byakuya would probably take that. For non-shinigami, DON KAN'OGI, BWAHAHAHAHAHA! Just kidding, Ishida is best.