Ichigo Kurosakivs Yhwach

He cuts Yhwach in half, which is the result the entire final arc is built toward and the only clean finish anybody gets against the tier above. It is second rather than first because of what it took to arrange: a blade reforged twice, a Quincy carrying stolen power, and the man who spent a hundred chapters as the previous villain holding the target still. Ichigo lands the blow. He does not do it alone and the arc never pretends he did.

He takes the Soul King out of the structure of the world and installs himself in the gap, which is the largest completed act anybody in this franchise manages. Before that he goes through a Gotei captain-commander who has been the strongest thing in Soul Society for a thousand years. His ability to choose which future occurs is the part to be careful with — it is explained far more often than it is demonstrated, and the explanations are doing work the panels do not.