For those who do not know Bleach, it is a famous anime based on the Shonen Jump manga of the same name by Tite Kubo. Bleachran for eight years, from 2004 to 2012, over 366 episodes and 4 movies. A perfect anime to binge watch if you are looking for something that will leave a lasting impression for a long time.
Going into detail, Bleachis about Ichigo Kurosaki, a teenager who can see and talk to spirits. His life changes when a Soul Reaper (Shinigami) enters his life. Soul Reapers are basically people who send wandering souls to where they should be after death and also cleanse Hollows (corrupted souls) of their sins. Their weapons are swords called Zanpakutō, which have unique abilities and powers. Bankai is the second stage of Zanpakutō power. It is considered one of the most powerful tools a Shinigami has.
Yes, Shunsui's Bankai name is a mouthful, but still, it's very effective and very strong. So strong, in fact, that Shunsui has to make sure his allies are at a safe distance before he activates it. Katen Kyōkotsu's powers envelop the enemy in a cold, dark and lonely space. Any wounds that are made on Shunsui's body are also reflected on his opponents (much like Naruto's Hidan), it covers the opponent's body in places that begin to bleed, it makes them feel unavoidable despair and pain. And finally, it allows Shunsui to cut the enemy's head with a wire attached to his fingers. Deadly and scary, this Bankai had to be in the top 3.
The aged and frail chief captain of all 13 divisions of the Soul Society. His appearance led many to underestimate him, but that quickly changed when his Zanpakutō powers were revealed. Ryujin Jakka is the oldest and most powerful type of fire Zanpakutō that has ever existed. In its Bankai state, it absorbs all the flames it produces in its previous state, and if cut, any opponent burns into nothingness. So strong that once activated, even the air and moisture of an entire area begins to dry up. If used for too long, this Bankai could destroy all enemies, the entire Soul Society and Yamamoto himself. Such strong power that the user must be careful not to go too far. Truly deserving of the number one spot.