Even Madara Uchiha, one of the most legendary Shinobi to ever exist, had to acknowledge the wrath and power of Might Guy after he opened his Eight Inner Gates. The whole Naruto series is filled with Shinobi who keep their trump cards hidden for the toughest times, and one such character is Might Guy.
He is showcased as an average ninja in most parts of the series which the audience might not take interest in making him in their favorite or strongest characters list. However, the Naruto series provided few references where it was hinted that considering him an average ninja might be a huge mistake.
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Who entitled Might Guy as the “Strongest of All” in the Naruto series?

During the Fourth Great Ninja War, Madara Uchiha, who is also one of the founders of the Hidden Leaf Village, was able to acquire the power of all Tailed Beasts and became the Ten-Tails Jinchuriki. When the whole shinobi world was losing hope, the ninjas who were considered to be the most powerful among others, including all nations’ Kages, both current and former, faced defeat.
Amidst the chaos, Might Guy jumped in and saved Kakashi, after analyzing and concluding that only Sage Jutsu and Taijutsu could work against him. Might Guy take charge and decide to battle against Madara Uchiha, as Taijutsu was his forte. Just after jumping into the battle, Might Guy opened his seventh inner gate, known as the Gate of Shock, which was found to be not very effective against Madara. In the bleak circumstances where every shinobi lost hope, that was when Might Guy unsheathed his trump card—the Eighth Inner Gate, known as the Gate of Death.
Might Guy went toe-to-toe with Madara after unleashing his life energy while using the Gate of Death. Madara was surprised and excited at the same time, as after Hashirama Senju (the First Hokage), Might Guy was the only one who put Madara in the toughest spot and made him this excited—said by Madara.
During their fierce battle, Madara acknowledged his powers and accepted that, among all those he fought in the past, there was no one who surpassed Might Guy in Taijutsu (physical combat). He then entitled Might Guy as the strongest of all.
Powers and Abilities of Might Guy and All the References in Naruto Which Suggest That He Is Not Easy to Deal With.

Might Guy didn’t inherit but learned the Eight Inner Gates technique from his father, Might Duy, who killed four of the Seven Ninja Swordsmen of the Mist single-handedly, according to the anime. Apart from having the Eight Inner Gates technique, Might Guy also possesses excellent physical combat skills and a turtle summoning.
The fact that he doesn’t belong to any reputed or powerful clan, lacks the ability to perform Ninjutsu and Genjutsu and solely depends on his physical strength. He builds all his strength by working relentlessly hard and, considering Kakashi as his rival, trained himself in such a way that he could fight without any problem with a Sharingan wielder.
So, when Itachi first appeared in the Hidden Leaf Village after Hiruzen’s (Third Hokage) death, he and his comrade Kisame handled Kakashi, Kurenai, and Asuma without putting in much effort. As soon as Might Guy intervened in the battle, Itachi stopped Kisame and warned him not to underestimate Might Guy. This reference also provides a brief hint from the Uchiha prodigy Itachi himself that Guy is not one who should be taken lightly, according to his demeanor.
Might Guy also proved his caliber in numerous instances throughout the Naruto series. A few such instances include his battles against the formidable Akatsuki member Kisame Hoshigaki. While the Raikage A and Killer B, who was also the Jinchuriki of the Eight-Tails, struggled against Kisame—also known as the Tailed Beast without a Tail—Guy made it look easier in their final battle on a secret island near Kumogakure (Hidden Cloud Village). Guy used the Seventh Gate of Shock to defeat Kisame Hoshigaki, who, in his final moments, ended up killing himself to prevent the secrets of the Akatsuki from getting into the hands of other nations.
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