Do you mean easiest to win against in battle or for the best battle?
Easiest: That fodder guy.
Best battle: Whomever has the best answer to long range attacks and around his strength level. This means it is a Mid-High-captain strength Soul Reaper.
Shunsui,Jushiro,All of Zero Division and Kenpachi are out of the Question (Yama and Aizen are out of his league too , but they're dead so mehh), This means that Byakuya or Rojuro are his best match depending on what Rojuro's bankai actually is.
Not in the Gotei 13.