While every bending form in Avatar: The Last Airbender is insurmountably powerful and useful, there's no doubt that some are just a little bit more-so than others. And though the talent of the bender is certainly a factor, there are still some forms that would ultimately prevail over the other despite bender ability.
With the expansion of The Legend of Korra, a wave of sub bending groups were introduced, and even fan-favorites like metal bending (invented by Toph in The Last Airbender) were explored further than the original series ever attempted. Viewers were able to witness a more diverse combination of bending battles, and with the changes that the new series brought to the bending game, there's finally enough substance for a fair ranking.
Airbending
Though Zaheer certainly expanded this element in one of the best airbending fights of The Legend of Korra, his mastery of air only goes so far, as it still remains an overall soft and gentle bending form. In "The Headband," Aang even tells his classmates that "The air nomads didn't have a formal military."
Airbending is about peace and spiritual freedom, not violence or power. When forced to be, of course, air can be quite powerful, as seen in "Sozin's Comet" when Aang blasts a gust of air so forceful that it breaks through an entire column of rock. But when compared to the other bending forms, it still takes the last spot, as the element's core is all about the diffusion of power and avoiding violence.
Metalbending
In Avatar: The Last Airbender, metalbending didn't exist until Toph invents it in one of her best episodes. She then primarily used her metalbending in practical situations, such as to bend the propeller of the ship in "Sozin's Comet," or break the door open in "Day of Black Sun."
And though in The Legend of Korra, metalbending is a much swifter and agile bending form, the introduction of unbendable metals like platinum quickly deter its practicality. A metalbender needs an available metal to bend, for one, and given that it's a much less accessible element than the rest, there's only so much power in the technique as a whole.
Waterbending
Katara is one of the most powerful benders in Avatar: The Last Airbender, holding her own against Master Pakuu even before she gained any formal training. In "Bitter Work," Uncle Iroh explains, "Waterbenders deal with the flow of energy. A waterbender lets their defense become their offense, turning their opponents energy against them."
Waterbending isn't about generating power, it is about transference, redirecting the existing power of the enemy back towards them. These benders work smarter, not harder, and it's why water is inherently "less powerful," though by no means any less capable,
Earthbending
As the most stubborn and hard element, earth can be quite powerful when needed. It's not a "fiery" bending form like lightning, lava, and combustion, and earthbending's stagnant and still qualities are the primary reason it doesn't pack quite as much of a punch.
In "Sozin's Comet," Bumi explains to Team Avatar how he took back Omashu during the solar eclipse, and a flashback reveals how he took down an enormous Ozai statue and moved entire buildings in his one-man quest to reclaim the city. These benders aren't to be messed with, as the manipulation of the literal earth is still quite a power to behold.
Lavabending
Although in practicality, lava is slow, sludgy, and not very easy to manipulate, it also has some pretty incredible destruction properties. Lavabending didn't get as much air time as the rest of the bending forms, since it was only introduced in season 3 of The Legend of Korra, but the episode "Enter the Void" proved its might as a sub-earthbending technique.
In the episode, Ghazan lavabends a portion of the Northern Air Temple, and in just a matter of minutes, the entire foundation of the temple is subsumed in lava. The whole building then collapses while a devastated Tenzin looks on. The bending form's power comes from this consumption quality, and it's really too bad that more earthbenders didn't possess the ability during the 100-year war in Avatar: The Last Airbender. It would have surely turned the tides.
Firebending
Of the base elements, firebending comes to the forefront simply due to the nature of fire itself. Destructive in its own right as a living, roaring force, fire can raze anything in its path, and adding the ability to manipulate this destruction is what makes the firebender so powerful. Ozai's one element stands toe-to-toe with Aang's four during most of their battle in "Sozin's Comet," and it's the capabilities of fire that made this equal footing possible.
It also doesn't hurt that while earthbenders and waterbenders need physical objects to manipulate, fire can be infinitely generated. Zuko even manages to firebend from his palms while underwater in "The Seige of the North," and in "Sozin's Comet," Jeong Jeong uses his bending to float in the air jetpack-style, proving fire's versatility as an element.
Lightningbending
Azula nearly succeeded at killing Aang with her lightning in "The Crossroads of Destiny," and the only reason he survived was due to Katara's special spirit water from the North Pole. And, if Zuko hadn't taught Aang to redirect lightning in "Sozin's Comet," he surely would have perished beneath the Fire Lord's terrifying and destructive blasts. Unlike fire, which can be blocked by the other elements, lightning can penetrate anything in its path, even shattering boulders if need be. Its power isn't all bad, though.
Since The Legend of Korra took place in a more mechanized and modern world, there was a greater need for electricity, and in "The Revelation," Mako shoots lightning into a large metal bolt to generate power. In that sense, lightningbending can be harnessed for good, and Korra helps to turn a new leaf for the previously villainized element.
Combustionbending
It's certainly a mouthful, but combustionbending deserves a top spot as one of the most powerful bending types. It's a major plot point in both Avatar: The Last Airbender and The Legend of Korra, and the two featured combustionbenders proved the might and terror of this sub-firebending technique. The unnamed "Combustion Man" in The Last Airbender and the electric P'li in Korra both possessed the ability to telekinetically shoot explosions from the third-eye tattoo upon their foreheads, and with this power, a combustionbender can precisely target anything they see.
Sokka put it the most eloquently in "The Beach" when the gang first encountered Combustion Man, as he exclaims, "How can we beat a guy who blows things up with his mind!" The short answer? It's really, really hard.
Bloodbending
With the right timing, skill, and place, nothing can stand in the way of a bloodbender. And with Yakone, Noatak (Amon), and Tarrlock's unique genetics that allowed them to bloodbend without the full moon, it's a truly formidable bending form. When Hama turns on Katara in "The Puppetmaster," she cackles, "I control every muscle, every vein in your body," and a terrified Katara helplessly swings through the air, proving Hama's point that bloodbending is the ultimate version of waterbending.
Had Avatar: The Last Airbender not been a children's television show, one could only imagine the full extent that a bloodbender could have used their power, and it's definitely one of the darker aspects of the bending world.
Energybending
Aang and Korra are the only benders shown to have this ability, and it's clearly a sheer and incredible responsibility. Aang uses his power to take Ozai's firebending in "Sozin's Comet," and Korra uses hers in "Endgame" to restore the benders whose abilities were taken by Amon.
It takes the cake over bloodbending because while Amon used bloodbending to block chi paths similar to Tai Lee's abilities, energybending goes beyond that, to the manipulation of someone's spirit and lifeforce. It's a staggering bending form because there is nothing physical about it, and what's more, only the Avatar has ever had the capability to execute it properly.