This... was a hard choice. I debated, between Zuko, Father and Azula, but one thing convinced me as to the right choice:
Note how Azula and Zuko define themselves:
Zuko is firstly son of Ursa, secondly brother of Azula, and lastly son of Ozai.
Azula is firstly daughter of Ozai, secondly sister of Zuko, and lastly daughter of Ursa.
That's the penultimate end to this chapter. And that, more than anything, tells me that the right choice is to talk about Ozai. Throughout this chapter there were other allusions, and I thought this one was just as powerful:
I think... I think Azula still isn't ready to confront who she wants to be. She has no idea who she wants to be. She's just broached the idea that she's not what she thought she was, that she isn't a monster. And so I think it's not yet time to think on what could be - I mean, she just spent this chapter trying to convince Zuko she should take the throne because she's the monster, and now she wants to go and think about what she could be? Before we do that, I think Azula needs to understand - who is her father, divorced from her, and why, and what's wrong with that.
[X] Your Father. In every life, they turn from him to save Zuko. Ty Lee has questioned him again and again. Mai has said you're 'weird' about him. But he's still—he's still your Father. The only one in the world who loved you from the start. Why is that supposed to be wrong?