Hmmm....well from the ones we have seen.....Yammamoto, Unohana, or Gin probably have the best Bankai.
Yammamoto: Zanka no Tachi's power basically eroded the environment. On top of that, touching him is virtually impossible. His blade basically one shots anything, and he can summing an army.
Unohana: Her Bankai is basically just as OP. Just by being around her you begin to melt away....and honestly in combat against her, its not like you could probably run away. It probably also contained many other unshown abilities.
Gin: The sheer speed makes his Bankai just OP....add on that a simply cut or stab csn implant his poison sword leads to basically death unless you're Aizen.
My personal favorite was Unohana's....just rediculously powerful.
