Fire Lord Ozai was the big bad of Avatar: The Last Airbender, and in order to bolster the drama of his position, he was frequently claimed in content outside the show to be the most powerful firebender in the world. This leads many fans to place Ozai in the top tier of any firebenders in the history of Avatar, from before the Hundred Year War up through The Legend of Korra. Unfortunately, his direct showings of power just don't support this.
Power and skill are intertwined in Avatar, and there are plenty more impressive fighters who perform feats Ozai was never capable of. While almost everything Ozai did was performed under Sozin's Comet, others during the comet had superior showings. Set aside the comet entirely and the case is clear: Ozai is far from Avatar's deadliest firebender.
Part of what's ridiculous about overhyping Ozai's abilities is that there is so little to base these on. Aside from a brief practice session bending in the comics and the lightning he threw at Zuko during the eclipse, Ozai has almost no firebending with which to gauge his abilities that wasn't affected by Sozin's Comet. To find a better firebender with just as few showings, fans need look no further than Sozin himself.
The Fire Lord of Roku's era really only had one episode to showcase his abilities, but virtually every time he did, they were impressive. He bested Roku in a sparring session when they were teenagers, and later attacked Roku with such a massive plume of flame that it's amazing he didn't have his own comet amplifying his abilities. Perhaps most memorably, he fought an entire volcano alongside Roku, heat-bending the lava with a technique never seen before or since. Virtually all the evidence points to Sozin being a better firebender than his grandson Ozai, showcasing more skill, raw power and unique talent.
When it comes to unique talents, it would be remiss to leave out combustion-bending. Whereas the original series' Combustion Man sacrificed agility and versatility for his unique art, his successor P'Li showed no such restrictions in The Legend of Korra. Even compared to the blasts of fire seen during Sozin's Comet, combustion-bending is far more destructive and precise in combat. At the same time, P'Li remained an impressive firebender with feats surpassing Ozai's.
Upon her release from prison, P'Li managed to shield herself and Ming-Hua from a burst of flame released by Zuko's dragon, Druk, right before knocking him out of the sky. She even proved she could exchange blasts of flame with Mako during their fight in Zaofu. Prior to her death, she held an entire team of metalbenders at bay. This all proves far more substantial in estimating her abilities than a vague statement about Ozai's power.
Perhaps a fairer comparison to Ozai's abilities would be how they stack up against others during Sozin's Comet, but even this comparison is not favorable. Alongside all the other firebenders in his fleet, Ozai's plume of flame he used to decimate Wulong Forest was no larger or more destructive than any of the others at his side. His legendary battle with Avatar Aang at that time provided an extensive look at the comet's true power, and yet it was really no more impressive than Azula's own showings during the same period.
If anything, the destruction of the buildings surrounding Azula's fight indicates she caused more damage than Ozai did. Ozai's flight was more proficient, but it's important to account for Azula's crazed state, deteriorating some of her control. She readily utilizes jet propulsion elsewhere in the series even without the comet, her lightning is repeatedly more destructive than Ozai's ever is, and in the comics, she even learns to redirect it as Ozai never did. Weigh the creator's statements that Ozai was the world's most powerful firebender against their statements that the blue hue of Azula's flames indicates the superiority of her power, and the scales tip toward Azula in terms of the evidence that she's the better bender.
In-character statements weigh almost as heavily as those made by creators, so many fans latch onto Iroh's uncertainty that he could defeat his brother in the finale to say that the elder brother is the inferior bender -- except that Iroh's statement is not necessarily any meaningful comparison of their powers. He says, "Even if I did defeat Ozai, and I don't know that I could..." which could indicate his moral reservations about killing his brother just as much as it could indicate doubt in his own abilities.
Regardless, Iroh's feats prove superior at every turn. During Sozin's Comet, he destroyed the wall of Ba Sing Se with his own fire blast -- a greater achievement of sheer destruction than most non-Avatars in the series. Ozai never came close to showcasing such power. Iroh also produced lightning that destroyed a wall during his hasty escape from the Dai Li, again showing superior destruction compared to Ozai's lightning during the eclipse. The clincher, of course, is that Iroh literally invented the technique of lightning redirection, giving him a unique skill that Ozai does not share, and which would be a hard counter to his little brother in an actual fight.
Sheer power, versatility, uniqueness and showings both influenced and uninfluenced by Sozin's Comet all provide meaningful metrics for asserting that other benders are superior to Ozai, and Jeong Jeong has a firm lock on all of these. During the comet, Jeong Jeong's flight was more precise than Ozai's, the sheer amount of fire he produced was greater, and the novel control he showcased in wielding those flames was far and away superior. No other firebending feat in the franchise comes close to Jeong Jeong creating columns of fire that tear through Ba Sing Se's streets, all while hovering in place midair.
Moreover, Jeong Jeong did not even need the comet to perform such astounding feats of firebending. In his debut episode, "The Deserter," he closed off an entire river with a wall of flame he raised on the spot, circumventing the usual need for firebenders to produce flame from their own bodies. No other firebender comes close to the level Jeong Jeong operates on, and that includes Ozai as much as anyone else.