Yachiru Unohana (Bleach) has mastered all the countless sword styles in the world, becoming the first Kenpachi.
Kenpachi Zaraki (Bleach) is one of the most powerful Shinigami to have ever lived. With his strength unleashed Kenpachi is able to slay Unohana, one of the strongest Shinigami of all time, with one blow.
Sosuke Aizen (Bleach) is one of the most powerful Shinigami in existence, the Wandenreich considered his power to be superior to Genryuusai.
Acnologia (Fairy Tail) who is considered the King of Dragons possess such ludicrous power that he is greater than the entire formidable armies of Alvarez and Ishgar and the mighty Wizard Saints and the dreaded Spriggan 12
Isaac Netero (Hunter × Hunter) is one of the strongest characters of the series and an extremely powerful practitioner of Nen.
Madara Uchiha (Naruto) was one of the most gifted shinobi in history. His tremendous skills in armed and unarmed combat allowed him to take an army of highly skilled ninjas at once.
Dracule Mihawk (One Piece) is the strongest swordsman in the world. His skills in swordsmanship are immense to the point where he is able to overpowering even Zoro with a mere dagger.
The Guardian (Samurai Jack) has countless eons of experience, and is the ultimate fighter, having beaten and nearly killing Jack with little effort.
Sigrun (Valkyrie Crusade) as the goddess of battle, has absolute combat skills.
Shinobu Sensui (Yu Yu Hakusho) is an incredibly powerful human who mastered the Resshūken, an elegant fighting style that is only available to fighters that have mastered all other forms of martial arts. He also mastered the Sacred Energy, the highest echelon of power that even Genkai couldn't achieve.
Raizen (Yu Yu Hakusho) was a Mazoku, an immensely powerful demon warrior, that even starving himself for centuries and merely using his descendant, Yusuke Urameshi, as a temporary medium he was able to decimate the mighty Sensui.
Ryu Hayabua (Ninja Gaiden series) has been stated to be the most powerful ninja in the world. His tremendous power and supreme fighting skills allow him to slay modern armies, vanquished demonic legions of supernatural monsters and defeat the strongest of fiendish deities.
Being of godly-status and having mastery in over 700 distinct fighting styles involving both armed and unarmed combat makes Raiden (Mortal Kombat) one of the most deadliest adversaries to come across of his Universe and perhaps the most versatile.
Meliodas (Nanatsu no Taizai) In the past was considered to be one of the most callous of the Demon Clan and the former leader of the Ten Commandments. Feared by even the Goddess Clan for his monstrous power, combat and sword skills. Being considered the only one worthy of the position of Demon King.
Layfon W. Alseif (Chrome Shelled Regios) gained the title of Heaven's Blade at the 10yrs old, even being considered to be a genius and can fight against contaminoids for several days. In the last volume of the light novel, his power greatly increased to the point where it surpassed eleven of the twelve heaven blades and is capable of filling an entire area with his own kei.
Kenshiro (Hokuto No Ken) ultimately regarded as the worlds strongest man. His mastery of the greatest martial art the globe over, The Fist of the North Star; has garnered him titles such as the Savior of The Century's End.
Jin Toujou (Shinmai Maou no Testament) has monstrous strength and abilities among the Hero Tribe, so much so that during their war with the Demon Clan, the original demon lord pulled back his forces in fear of Jin's monstrous strength; being of akin to a War God.
Nagi Springfield (Negima) who is considered as the strongest "Magic Knight" Magi not only for his magic prowess, but also monstrous battle strength. In just his teens defeated the Mage of Beginning, a powerful and Ancient Archmage feared as the greatest threat to the Magic World.
Jack Rakan (Negima) a powerful Gladiator with immense combat strength, magic and ki that borders on insanity, being the only person who could fight against his eternal rival, Nagi Springfield. His abilities earned him the titles 'Thousand Blades', 'Man Who Cannot Die', 'Immortal Fool, 'Human Atomic Bomb', 'Ultimate Hard Worker', and 'That Damn Guy Who You Can Stab With Swords All You Like And'...
...I'm sure you get the point.
Esdeath (Akame ga Kill) is the strongest warrior of the Empire, it took the combined military might of one million men and ten Teigu users including Akame to finally defeat her.
Yonko Charlotte Linlin (One Piece) is one of the most powerful pirates; even at the age of 5, she effortlessly defeated one of the Giants' greatest warriors.
Kaido (One Piece) is hailed as the strongest beast in the world.