Who do you think is the best firebender in Atla?
Many people believe Iroh is the most powerful firebender simply because of his fandom. And honestly, I highly doubt he is because he doesn't have many feats, and his only "great feat" is protecting Ba Sing Se. But guess what? A comet increased his power, so I don't think it's due to his own merit.
So no, Iroh is not the most powerful firebender in Atla.
Ozai? In the series, he's canonically the most powerful firebender in Atla, but his only "feats" are defeating his own son, who was on the ground begging for forgiveness, and beating up another child. But oh... it turns out he also had the comet boosting his power, so it's not entirely his own doing.
Maybe the creators said he's the best firebender, but I doubt it. It's probably in the top four if we consider Sozin and Azulon.
(And we have to remember that Aang took away his firebending.)
Maybe Zuko??? Nah, maybe he's the best of the worst/average, but he's the worst of the best/prodigies.
I mean, Zuko's only major feat was "defeating" his prodigious sister, but I doubt that counts because she was at her worst, and his fire was amplified by the comet.
So no, he's not Atla's best firebender either.
Azula? Yes, I think she's definitely the greatest firebender in Atla, and of all time.
Am I speaking out of fanaticism? I don't know. I'd like to know of a firebender much better than her. Is there one? No, there isn't.
She's probably the only member of the royal family who has achieved so many feats on her own merits and without needing a comet to boost her power.
Who can we compare her to? Iroh, Ozai, and Zuko? They're barely reaching their heels right now.
Sozin and Azulon? Phew, I hope they can fight for the title of greatest firebender in their graves.
Jeong Jeong, Rangi, or Mako? They were defeated very easily by others, so I doubt it.
So... Answering your question, it's definitely Azula.