What was the most important scene in The Absolutely True Diary of a Part-Time Indian?

Any number of scenes could provide a variety of answers to
your question, but I'll offer one.

Junior's first day at
Reardon is certainly an important event and transformation in the novel. In this scene,
all of the apprehensions and anxieties of crossing the divide from one culture to
another come to fruition. Junior is initially ignored by the white students but the
taunting and threatening remarks soon follow. Of course, these tensions continue to boil
within Junior an eventually he is driven to violence is
retaliation.

So much of this novel is about living caught
between two radically different worlds. This scene when those tensions become a physical
reality is certainly one of the novel's most important.