Yuki Yuna is a Hero's third season began in early October with the episode The Joys of Youth. It was a mostly happy episode, but knowing this show, that happiness couldn't last. It was only a matter of time before the girls realized that something was off and were subsequently forced to take up their Hero mantles again. The Heroes fight against nearly indestructible enemies called Vertex that try to invade their world. As part of the Dark Magical Girlanime subgenre, Yuki Yuna is a Hero shows a terrible side to power.
The most powerful characters are often the ones who suffer the most. In this case, Heroes can activate a powerful form called Mankai at the cost of one of their bodily functions. The consequences of going Mankai are called Sange. Some Heroes are strong due to their bravery in choosing to activate Mankai, and some are strong for how hard they fight without it.
Spoilers for the anime will be included.
6 Miyoshi Karin
Karin first swoops in during a fight and destroys a Vertex singlehandedly. She then transfers schools and joins the Hero Club. The arrogant and abrasive Karin was trained to be the greatest Hero with the help of combat data from past Heroes. She was sent by the Taisha, the religious group in charge of the Hero system, to oversee the Hero Club. She was unhappy with their happy-go-lucky behavior, which she believed showed that they weren't taking things seriously.
After a battle in which all of them activated Mankai but her, Karin realized she was wrong about them and ended up becoming very close with the other four girls. Karin's main weapons are katanas that she dual-wields and can detonate. In Mankai form, she gains four giant robot arms with katanas. Karin can throw huge volleys of explosive katanas and shoot a giant beam capable of destroying a Vertex in one hit.
5 Togo Mimori/Washio Sumi
Togo, who lost her memory and legs in an "accident," is a unique case. When the Hero Club first transformed, they gained their weapons and powers from fairies, which also shielded them from physical harm. Togo was the only one with three fairies and multiple weapons. It turned out that it wasn't Togo's first time as a Hero, and her two extra fairies were due to activating Mankai twice in the past, losing her legs and memory in the process. Her original name was Washio Sumi and her original weapon was a bow. After an upgrade to the Hero system, it changed to a gun.
As Togo, she has multiple guns including a powerful sniper rifle. After activating Mankai again, she gets drones that automatically fire on enemies. She even came close to defeating the most powerful Vertex, the Leo Vertex. Togo's Mankai grants her a giant gunship with powerful cannons.
4 Koori Chikage
Chikage was a Hero much earlier than Togo and Karin. Her weapon was the Death God's scythe, capable of destroying lesser Vertex in seconds. Before the Mankai system, Heroes used Trump Cards to increase their powers by channeling mythological beings. Using Trump cards caused a physical and mental backlash.
Chikage was able to use Trump cards to create seven copies of herself that regenerated. She couldn't be killed unless all seven copies were destroyed at once. While not as powerful an attacker as some of the other Heroes, her endurance and ability to keep coming back make her a daunting opponent. Even after Chikage was stripped of her Hero powers due to becoming irrational and attempting to kill a teammate, she still managed to protect that same teammate by taking an attack from a Vertex.
3 Minowa Gin
Gin was one of Togo's original teammates, back when she was Sumi. Her main weapons were axes that she dual-wielded which could generate and manipulate fire. Gin's defining moment came when she, Sonoko, and Sumi fell into a trap. Trump cards were gone and the fairy system hadn't been implemented yet, so the girls took the full damage of the attack.
With Sumi and Sonoko injured and out of commission, Gin was the only one capable of continuing to fight. Determined to protect her friends, Gin managed to drive off three Vertex without Trump cards, fairies, or Mankai. She took a fatal amount of damage and only finished the fight through sheer willpower. Her sacrifice is what drove the Taisha to implement the Mankai system and introduce fairies.
2 Yuki Yuna
Well, the anime is named after her for a reason! Yuna has the highest score ever recorded on Hero aptitude exams. Her weapons are gauntlets which, mixed with her incredible strength in Hero form, do massive damage with every punch. She was even capable of throwing the Leo Vertex a long distance. Yuna also has an explosive kick that destroys everything in the surrounding area when it makes contact.
Yuna's Mankai form gives her giant floating metal arms to punch with. During the final battle of season one, she put so much effort into taking down the last Vertex that she went comatose. It was later revealed that Yuna destroyed her entire body and had it replaced by a magical construct. Yuna's also the only Hero with the ability to activate Dai-Mankai, or Great Mankai, as a being favored by the Gods. In that form, she's able to destroy the World of Flames that was outside the protective barrier created by the Earthly Gods.
1 Nogi Sonoko
Sonoko was teammates with Sumi and Gin and was the last one standing after Gin died and Sumi lost her memory. Her main weapon is a spear that doubles as a shield and her Mankai gives her a giant warship with spears for oars. Sonoko is also highly intelligent, tactical, and calm under pressure. She can move fast enough to cause shockwaves.
In her final battle, Sonoko activated Mankai 20 times and defeated 12 Vertex including the Leo Vertex. The Taisha worship her and consider her the strongest Hero. Since she now has 21 fairies, she has more powers than any other character. What's most impressive about Sonoko isn't her powers or weapons, but the lengths she went through and the things she sacrificed to protect her friends and the world.