To answer your list Im surprised you have Ozai above Zaheer, I feel like Zaheer was way more relatable in what he wanted to do tbh. Plus his fights were cooler.
Zaheer probably had more screen time than Ozai and definitely was fresh in motives and personality. But Ozai is Ozai man lol.
He treats Zuko like a bastard to spite his wife over notes she was trying to send to an old lover.
Is willing to kill Zuko when Azulon demands it.
He proceeds to have his wife poison his dad then tells her to leave or he'll kill Zuko.
Scars Zuko and sets his character up for all of the show.
Performed amazingly by Mark Hamill.
Even when he's not seen, like in the first two seasons, Ozai kind of is the Fire Nation and every evil thing we see them do is because of him. He's so integral to the story, I can't really put anyone above him, despite how the showrunners opted to exclude him from most of the series.