1) Kisuke Urahara- Kisuke Urahara is one of the most intelligent shinigami of the soul society and one of the strongest also. He sealed Aizen with a new sealing technique that he himself invented. He seened the Soul King before and he helped Ichigo kuasaki train.He has many tricks up his sleeve and has a shady side also.Kisuke Urahara is really OP!!!
2) Sōsuke Aizen- One of the most powerful soul reapers of all time. He betrayed the soul society and was able to master the Hōgyoku by merging with it. He also saw the soul king like kisuke urahara did and he question kisuke by saying," you've seen it too, and why would you help?" Kisuke responded by "being quiet and nodding his head".Sosuke aizen also made arrancars and was behing Hollowfication.Aizen is also a master at Provication( Manipulating) and hes awfully intelligent.