The Legend of Korra introduced one of the deadliest Benders in Avatar with Amon. Gifted with the ability to bloodbend even without a full moon, the series made sure to establish that Amon was one of, if not the most powerful, Bloodbenders in the franchise. In defeating him Team Avatar had to use the full range of abilities available to them, and develop new ones on the spot just to counter the deadly skill.
And yet Amon could have been even deadlier. A subtle moment from the series points to bloodbending's best technique, but it's not Amon who uses it. Amon may have been more powerful, but it seems his brother Tarrlok may need more credit for utilizing one of the forbidden art's most effective options in combat.
When bloodbending was first introduced in Avatar: The Last Airbender, it came with a severe limitation that required a full moon for it to be used. Waterbenders could control the water inside a person's blood to puppeteer them, but only when they had the power of the full moon behind them. Even then, only its inventor Hama and her would-be student Katara knew the ability. But The Legend of Korra introduced a family with an inborn ability that allowed them to use the skill at any time they wanted. Yakone could paralyze an entire courtroom of people to escape his trial, while his son Tarrlok used the ability to defeat Korra in single combat. Intended to be even deadlier than either was Amon, shown mastering the ability like a prodigy from early on.
Though he posed as a Nonbender, when Amon's secret was revealed and he unleashed the full power of his bloodbending, he proved to be one of the deadliest villains in all of Avatar. Without even moving he could render his opponents useless in combat, and yet even with such an extraordinary ability, there were two occasions where his influence was overpowered. In the first, he was against Mako, who unleashed a bolt of lightning that broke Amon's concentration on his target, and in the next Avatar Korra used her airbending for the first time to send him flying out a window. These moments reveal imperfections in Amon's tactics, and though the series demonstrably proves his superiority to his younger brother Tarrlok, there is actually a moment where Tarrlok showcases a talent that could have made Amon an even deadlier Bender: knocking his opponents out.
The showing comes in Episode 9 of Book One when Team Avatar confronts Tarrlok after Korra's disappearance. Informed that Tarrlok is actually a Bloodbender, Tenzin, Lin Beifong, Mako and several others suddenly attack Tarrlok to try and put him down. But before any of their attacks can land, Tarrlok paralyzes and pushes them to the ground with his bloodbending. Tarrlok then does something no other Bloodbender has ever done: He knocks them unconscious. Tarrlok escapes and, when we next see Team Avatar, Tenzin is still unconscious on the floor as Lin slaps him awake.
So much is happening in the moment that it's easy to overlook, but Tarrlok's technique here is probably the most broken ability in all of Avatar. An unconscious foe is even more useless than one actively paralyzed with bloodbending, and as Mako and Korra's showings against Amon demonstrate, even the active hold of bloodbending can be broken with enough willpower and focus. Had Amon simply knocked them unconscious with bloodbending they would have been totally at his mercy. His neglecting to do so is a subtle instance of one of Avatar's most recurring themes: that power isn't everything if its wielder doesn't know how to use it effectively.
There is little question that Amon is more powerful than Tarrlok, as his attempts to bloodbend Amon fall flat later on when the elder brother simply pushes through his influence. Even in Tarrlok's own account of their childhood, it was always Amon who had the greater proclivity for the talent, but it seems Tarrlok was selling himself a little short. Even against half a dozen people, some of whom are among the strongest Benders in Korra, Tarrlok managed to easily disable all of them and escape.
Bloodbending is already one of the strongest abilities in Avatar, but this specific technique is doubtless its most effective option in any combat scenario. With both Tarrlok and Amon dead by the end of Korra's first season, and the seemingly genetic ability lost with their bloodline, there may not be another character who can use the technique in the future. Bloodbending's best move likely died with one of its most effective users, and as much of a surprise as it may be, it was never Amon.