As far as design goes, Tenza Zangetsu is the best for me, but of course Yamamotos bankai is the best not just because it's OP, but because it covers him defensively too, he's pretty much untouchable in bankai..
Best Bankai would have to be Yamamoto's Zanka no Tachi. That's not debatable. Following after it COULD be Aizen's bankai if he had one or maybe Isshin's or Urahara's.