It's possible that Azula from Avatar: The Last Airbender would have grown up to become the greatest Firebender to ever exist, suring even her father, Ozai — if only she didn't allow herself to fall prey to her own paranoia. In any case, assuming peak mental stability, there are very, very few characters who could hope to take her on and come out alive (but certainly not unscathed.)
Her Lightningbending, especially, is dangerous because it is both powerful and volatile, not to mention that she can produce blue-colored fire, one of the rarest varieties possible. Therefore, it isn't much of a stretch to assume that Azula would find most benders in The Legend of Korra to be easy targets; the list offered here provides ten such examples.
Bumi
Bumi might be nearing retirement age, but his recently discovered Airbending skills are nothing to be trifled with (not after he trains under his little brother.)
Further, he has a tendency towards employing utterly ludicrous tactics in combat, for instance, when he accidentally rips through the Northern Water Tribe military camp and knocks out Eska and Desna in the span of minutes. Nevertheless, a single lightning bolt from Azula would be able to stun him into submission, if not kill him outright.
Tonraq
The Avatar's father is no average Waterbender, but he is certainly not powerful enough to win against extremely dangerous villains like Azula. This is because his style of combat is structured towards efficiency, which means that his moves can be read from miles away.
On the other hand, the Fire Princess's techniques are as unpredictable as her behavior: she might kick up a firestorm, or she might assassinate Tonraq without anyone having realized that it had happened. The deception is strong with this one.
Opal
Opal, Lin Beifong's only daughter, is a new but talented Airbender, even being praised by Korra for rapidly developing her skill set. She has been known to end duels between highly accomplished Earthbenders (her aunt and mother, respectively) but it must be noted that she was enraged at the time.
Azula might have to try a little harder in this case, but a carefully orchestrated Firebending strike would take Opal out of commission before the latter even notices the blue blaze hurtling towards her.
Ming-hua
Ming-Hua's battle with Mako is legendary, but will probably not go down in history simply due to lack of evidence (or witnesses.) Her physical disadvantage makes no difference to the sheer amount of innovation in her Waterbending; in fact, she becomes more terrifying the more water she manages to accumulate around her body.
Azula could technically take Ming-Hua with the same old method, electrocution, but she'd probably go for a less final move: if only to keep the match more interesting and active for a bit longer.
Kya
Although in her fifties, Kya shares almost as much aptitude for Waterbending as Katara, in of both intensity and finesse. She has even been able to (for a short time) counter opponents as overwhelming as Zaheer, using a range of stances to keep the master Airbender at bay.
Azula, however, rarely loses stamina during her matches, given that her emotional distress is the source of her bending; in addition, she is quite creative with her Fire- and Lightningbending, and is far too agile for Kya to catch her off-guard in any situation.
Wing & Wei
The twin boys of Zaofu, Wing and Wei Beifong, are proficient Earthbenders (probably those Toph genes), but also possess a finely-honed knack for Metalbending. While together, their synchronous modes of attack/defense are nearly impossible to overcome, so much so that it takes the Avatar to beat them at their own game: Power Disc.
All Azula needs to consider here is the fact that metals make amazing conductors of electricity, and neither Wei nor Wing would make it out alive. And certainly not in one piece.
Bolin
Bolin is one of the best characters in both the shows — his bubbly charm and tender masculinity are inspirational, to say the least. This doesn't mean that he isn't sharp in a fight, and his internal aim helps him land a hit against the tiniest of targets — the most extraordinary being a perfect strike against the P'Li's forehead tattoo in order to incapacitate her (at least temporarily.)
Bolin's discovery that he can Lavabend is both awesome and overpowering, but the slowness of his bending style will not hold up against Azula's Lightningbending blasts.
Tenzin
Tenzin is not one to give up (or give in, for that matter), especially when his nearest and dearest are in danger. Having studied under the Avatar himself, his Airbending is among the best in the world, even managing to almost defeat Zaheer in battle; until his pals come to his rescue (who also have to give it their hardest to overwhelm Tenzin.)
Azula would also be victorious against the Air Nation leader, but only because he strictly follows the philosophical regimen prescribed by his tribe: the avoidance of murder at all costs.
Kuvira
Unlike Korra, whose Avatar State fails when she is about to kill Kuvira, Azula would show no such psychological failure. Kuvira's Earth- and Metalbending repertoire is as varied as it is ruthless, given that she shows no hesitation in of taking her enemies down by force or by threat.
It won't matter against the Fire Princess, because she would easily evade the gigantic blocks of mud being tossed her way, probably even using them as literal stepping stones to get closer to Kuvira, and then... an unblockable inferno at point-blank range.
Mako
Mako, Korra's on-again, off-again crush, is one of the strongest Lightningbenders in the Avatar multiverse, considering that he has the capacity to defeat a three-member Pro-Bending squad without any backup from his team.
He also wields succeeds in distracting Amon by Lightningbending while under the influence of the villain's Bloodbending, greatly inspiring the Equalist leader for his audacity. Nonetheless, it can be assumed that Mako vs. Azula would end in the arena being vaporized, but this certainty can also be applied to his fate — a smoldering mass of charcoal where he is supposed to be standing.
