Avatar: The Last Airbender is a universally renowned show and gave audiences a plethora of phenomenal characters to not only root for but also root against with several deadly villains.
The show's spin-off, Legend of Korra, may not be as loved as it's predecessor, but one thing almost all fans of the two shows can agree on is that the villain's Korra had to face were far deadlier and more challenging than the ones Aang had to take one. Regardless of who was the deadliest of them all, fans more than likely have one specific villain as their favorite - and whichever one that is may say more than they realize.
Combustion Man
Combustion Man was quite possibly the scariest thing to come out of ATLA - he could firebend with his mind by sending a sonic-blast of fire out of his forehead. Even the most skilled benders stood virtually no chance against this unique bender and it took Sokka's trusty boomerang to finally bring him down.
If someone has Combustion Man as their favorite character, there's a chance that they're a hot-head, but it's also possible they're a person who merely uses few words and more action to get through any obstacles in your life.
Long Feng
Long Feng may have stood no chance against Azula (few did) but that didn't negate the fact that he held control over the largest city in the world for quite some time, all the while feeding lies to the people and kidnapping/brainwashing anybody who tried speaking about the war within Ba Sing Se.
If one has Long Feng their favorite villain, they're less likely to try and bring down their enemies with force and fighting and would rather try a cunning, manipulative approach in order to stick to the shadows and keep their hands 'clean'.
Admiral Zhao
Zhao was quite the formidable foe for season 1 and quickly helped elevate the stakes as he tried to capture Aang while also constantly getting in Prince Zuko's way of doing the same. Despite his failure to become the Moon-Slayer, it was his actions that ultimately caused Yue to sacrifice herself.
If one has Zhao their favorite villain, chances are that they're a little power-mad, constantly chasing and craving more and more while never quite appreciating the things that they already have.
Red Lotus
These guys were on another level. Quite a few people seemed to have a problem with the Avatar during Korra's era and this group was arguably the deadliest - a combustion bender, a lava-bender, and a no-armed water-bender (among others) - all trying to complete the task of eliminating the Avatar.
If one has the Red Lotus members as their favorite villains, they may just be a person who appreciates unique/creative aspects of otherwise ordinary things. Or they could potentially be a member of a villainous-cult...
Vaatu
When it was revealed that the Avatar was actually a person combined with a light-spirit (named Rava) it only made sense that there would be a dark-spirit that would be Rava's opposite - that gave us Vaata, a dark-spirit that had been locked away for 10,000 years by the very first Avatar.
If one's favorite is Vaatu, then it's probable that they're a person who understands that everything in life has a balance - there can't be light without darkness, and vice versa.
Azula
It's hard to believe that Azula wasn't fully introduced until season 2 - she was that scary, making a lasting impression as one of the most sadistic villains in either of the two shows and still the only bender ever to produce blue fire.
If one's favorite is Azula, perhaps they're a person who is perfectly capable of inspiring a room full of people to follow them - just be careful that fear isn't the primary motivator of people's desire to offer their allegience.
Ozai
Ozai was just evil - plain & simple. Following in the steps of his father and grandfather, everything about this dictator was encompassed by his sole desire to bathe the world in fire, only stopping when Aang finally took his bending away for good.
If one's favorite is Ozai, they might be a person who craves power and is capable of using a combination of force, inspiration, and (most importantly) fear to get people to back their ideas/visions.
Unalaq
Unalaq is a villain that doesn't quite get the credit he may be due. Not only was he a formidable water-bender but he also had the ability to spirit-bend - and that was all before he fused with Vaatu and became a Dark-Avatar.
If one's favorite is Unalaq, they might be a person who is just as powerful/capable as the other people around them, though they potentially fall through the cracks due to ambiguous endings.
Kuvira
Ozai would've liked her. An evil dictator in her own right, Kuvira did things slightly different than Ozai by seeming like a peaceful-ruler at first, then quickly revealing that she was just another power-hungry warlord who was keen on seizing control of the whole world.
If one's favorite is Kuvira, it's possible they're a person who appreciates longevity (or the long-game) meticulously plotting out strategies and defeating enemies with cunning intellect (before resorting to forceful tyranny).
Zaheer
The one true villain air-bender that either of the series had was beyond scary, already having been locked away in solitary confinement for 13 years (and that was before he received his air-bending skills). Not only that, but Zaheer was the leader of the Red Lotus, arguably the deadliest group of benders in either of the series.
If one's favorite is Zaheer, there's a chance that they're a person who is very spiritually in tune with themselves and their thoughts are very structured - however, those thoughts could turn into extremes.
Amon
While Hama, Yakone, and Tarrlock were all terrifying blood-benders in their own right, Amon easily took the crown as scariest of them all. The only bender (other than the Avatar) who could take other people's bending away, mixed with his psychic blood-bending techniques and the fact that he didn't have to bend for the entire season he was in creates a truly deadly villain.
If one's favorite is Amon, it's probable that they're a person who understands the nature of evolution/change and that things can't always stay the same. They don't let anything deter them from striving for their goals.