2. Soul Society Arc
Possibly Kubo's magnup opus, and for good reasons. The conflict was gray, and most of the fights connect to the theme of duty, how far people go in order to fulfill it, is it always right to abide to it etc. There are little sprinkles of this everywhere through the arc. Ikkaku starting to like Ichigo, even though he should kill him in spot, Kyoraku and Ukitake turning against Yamamoto, Ichigo defeating/convincing the gatekeeper to open the gate, Tousen-Komamura (altough theirs is more about honor specificaly) etc, complete "antithesises" to it, Aizen's betreyal, Kenpachi not really giving a shit. And of course the big events, Ichigo vs. Renji, Ichigo vs. Byakuya, Renji vs. Byakuya and Soi Fon vs. Yoruichi. All of these fights have, on one side someone who wholeheartedly believes that people always come before duty (Yoruichi is kinda chill about it), while on the other hand we have obviously the opposite, but not complete opposite. Byakuya, Renji and Soi Fon all had doubts, especially Renji. He was the one that was showing his denial the most, and yet he realised that his resolve to uphold his duty was weak, against the insurmountable strongheaded Ichigo. Byakuya was the most stubborn one, which only made his acknowledgement of his own loss that much greater. Byakuya clearly had enough energy to finish off ichigo, yet he didn't. Because he acknowledged Ichigo's resolve and realised his own true desire, to let Rukia live. He took the L like a boss. As a bonus, the moment Rukia stabbed Ichigo in the first chaper, has now even more grounding than it initialy had, because she threw away her duty as a shinigami to help Ichigo. And Ichigo came back for her to repay that dept.