One of the best things about Avatar: The Last Airbender was its diverse array of villains. For such a smart show, it was able to showcase a lot of smart villains, and the smarter they were, the greater threat they posed.
So here's a rundown of the 10 smartest villains that Team Avatar had to come across through their long quest to save the world from the Fire Nation's tyranny. It's never a bad time to talk about Avatar, but with its recent release on Netflix, it's a hot (pun intended) topic!
Combustion Man
The silent and mysterious mercenary who Zuko enlists to hunt down and kill Aang in Book 3 is a pretty terrifying figure. A big buff guy with metal prosthetics (presumably from his own explosions gone wrong) and the ability to project explosions from his head. He doesn't get to be too high on this list, seeing as he blew himself up a few times (and explosion which led to his demise), but the fact that he was able to discover such a deadly new ability and be a successful mercenary for years means something.
Long Feng
The corrupt and scheming leader of the secret police of the city of Ba Sing Se, Long Feng and his Dai Li agents had been in control of the city for years. Though the person they were fooling (the king) wasn't the sharpest tool in the shed, the fact that they were able to control such a massive city was an impressive feat. Long Feng was a pretty cunning and skilled strategist, engaging in political schemes, brainwashing, and deceit, but ultimately, he was utterly humiliated by Azula after trying to reestablish himself as a big player.
Ozai
Fire Lord Ozai is the big baddie of The Last Airbender, as the sitting head of the Fire Nation. He was a cruel and manipulative figure, responsible for much of the grief that Team Avatar would suffer under people following his orders.
He was also a skilled combatant, able to fight Aang viciously for some while. Like the Fire Lords before him, he held his power through aggressive conquest and ruling through fear, and had it not been for the return of the Avatar, Ozai would have succeeded in his world conquest.
Sozin
Fire Lord Sozin was the instigator of the Hundred Year War, which is a pretty terrible legacy to leave behind. However, as ruthless as his strategy and actions were, they did ensure that his nation was the alpha for an entire century. He was smart enough to take advantage of the impending comet (later named after himself) to increase the bending ability of his people and wipe out the Air Nomads, in an attempt to kill the Avatar.
And indeed, he was wise to realize the Avatar was the greatest threat to his power and left the previous one to die, his own best friend. His strategies were brutal and downright genocidal, but he did manage to accomplish most of his goals and stand virtually unopposed.
Ty Lee
Ty Lee has a ditsy and carefree attitude, but that doesn't undermine her intelligence. Ty Lee was actually a very deadly and perceptive opponent in combat, with the ability to temporarily block people's bending powers through precise strikes and fighting techniques. She was also decently tactical from time to time and able to assess situations quickly. She also was smart enough not to accept Azula's offer to join her crew initially, though she was coerced into joining later.
Mai
Mai is a cynical and seemingly apathetic person, but that's because she thinks herself as above the stupidity of all the events and people around her. And to some extent, she's right. Her self-awareness and aloofness are signs of her reluctance to take part in the Fire Nation's evil actions, but she's so cynical that she just sort of goes along with it when Azula asks her (but only for so long). Mai is a pretty fierce fighter, just like Ty Lee, but she is a bit more focused in reading people and in plotting, so she gets a slightly higher edge.
Hama
Hama's fall to villainy is pretty tragic but understandable. After her village in the Southern Water Tribe was raided by the Fire Nation, she was left imprisoned for being a water bender. While she spent years in prison, she became very imaginative with how to use her water bending, and eventually realized blood has water in it. She began to blood bend and was able to break herself out by controlling the guards like puppets. Her discovery of the chilling new craft is akin to Toph's discovery of metal bending but is arguably even more effective, albeit taboo.
Zhao
Admiral Zhao was a very cunning man, but he constantly undermined himself (like many fire benders) with his angry outbursts and impulsive decisions from those bouts of rage. Zhao was pretty well trained in his fire bending by a great teacher (Jeong Jeong The Deserter), but left due to his impatience. However, he did seem to have more focus in book learning, going so far as to study for some time in Wan Shi Tong's Library, and was able to discover the secrets on how to defeat the Water Tribes and their hallowed spirits, though as always, via ruthless means.
Zhao was also able to easily read Zuko's lies and was just as capable of tracking down Aang, and earned his title of "admiral" through his great strategy. Had it not been for his arrogance and temper, he could have been an even more serious threat than he already was.
Azula
Azula was a sadistic and terrifying villain, constantly chasing perfection to prove her worth to the world. And to be fair, she did frequently deliver on that front. She was a tactical genius, a perfect liar, a crafty manipulator, a deadly fighter, and never left room for failure. Ultimately, that would be her downfall, because rather than learn from failure, she would obsess over the fact that she had failed, and it drove her mad. But until her breaking point, she had been the greatest thorn in the side of Team Avatar, thwarting their plans and crushing their hope at every waking moment.
She was by far the most valuable asset to the Fire Nation, and was even able to kill Aang while he was entering the Avatar state, potentially not just killing the Avatar, but breaking the entire cycle. She knew what she was doing, and she felt confident in showing off to the rest in how good she was at doing it because she knew they didn't pose a threat. Her greatest enemy was herself.
Zuko
Zuko was a pretty tortured soul and was often, like most fire benders, impulsive and rash at times. However, Zuko had a good heart underneath his sinister exterior, and it was always evident that he was greater of doing so much more than his father's bidding. While he was a pretty effective villain, always managing to track down Aang at any corner of the globe and a skilled combatant, it was his gradual turn to the light that made him intelligent. True wisdom is realizing that genocide and conquest aren't a good way to live life. And listening to his wise Uncle Iroh eventually helped push him to make the right and most intelligent decision any villain on this list could make: Stop being a villain.
It was this decision that would prove to be the downfall of the Fire Nation, as Zuko's actions would help lead to the downfall of his father and Azula, seeing as he was able to finally give a tactical edge to Team Avatar and teach the Avatar real fire bending, having learned from his own mistakes and misgivings.