Most likely Ozai because he became good in book 3 and join the Gaang where they killed Fire Lord Zuko !

Technically Zuko was the Phoenix King at that point. Do you mean Fire Lord Mai, from whom Ozai saved Sokka by taking the knife she had thrown at him? She was also killed, if I recall correctly. Though, the kill really only belonged to Sokka, since he was the one that shot her using his iconic Bow and Arrow.

No fanfiction, thank you.

Who voted Ozai? He's an abusive father to his kids. Zuko (who is my second favorite ATLA character, Sokka being first) could have acted like he did in the first few episodes of season one the whole show and he would still be better than his father.