This was a very close competition between Senbonzakura Kageyoshi and Zanka no Tachi.
They are both absolutely incredible and have made some of Kubo’s best work so much better yet. Zanka no Tachi is supremely powerful and the four cardinal directions with their respective techniques are awe inspiring.
But Senbonzakura Kageyoshi is also one of the most powerful Bankai among the Gotei 13, and on top of that it is arguably the single most elegant and bewilderingly beautiful zanpakuto in the entirety of Bleach. It is very diverse, with a plethora of techniques, each more deadly than the last.
It’s one of the best explored abilities in the series and is the highlight of any situation it’s involved in.