The Fourth Hokage
Minato Namikaze, the Fourth Hokage, is the legendary Ninja who defeated the Kyuubi nine-tailed fox, which resulted in the demon being sealed inside Naruto. Minato possessed immense taijutsu skills and was famous for his incredible attack speed, which is how came by his nickname, the "Yellow Flash." He was also a master of ninjutsu, originator of the Rasengan, a user of space/time jutsu, such as Madara uses, and possessor of powerful sealing techniques, such as he used to seal the Kyuubi..
Madara
Uchiha Madara is perhaps the most powerful antagonist in the "Naruto" universe. The real mastermind behind the Akatsuki criminal organization, he was a rival of the First Hokage and has mastered a type of immortality jutsu. Since Pain's death he has claimed the Rinnegan eye from Negato to complement the Sharingan and can thus utilize the Six Paths of Pain, as well as the illusion and hypnosis abilities of the Mangekyou Sharingan. He has also mastered space/time jutsu and is able to summon the Demonic Statue of the Outer Path.
The Third Hokage
Hiruzen Sarutobi, the Third Hokage, was one of the strongest Ninja ever to be born in Konoha village. Known as the "God of Shinobi" according to Iruka, he was the strongest of all Konoha's Kages and he demonstrated this by managing to hold his own against Orochimaru and two former Hokages, even when he was well past his prime. In his battle against the villain, he displayed a mastery of taijutsu, using his diamond staff, the weapon form of his Monkey-King summon Enma, to great use. He also used powerful elemental attacks.
Pain
Pain, previously known as Negato, was the leader of Akatsuki and the only person in the world to possess the Rinnegan, the most powerful eye technique inherited from the Rikudo Sennin, the Sage of the Six Paths. His Rinnegan enables him to use the Six Paths of Pain, based on the Buddhist paths of Reincarnation, which allows him to control six different bodies, each with unique powers. He also possesses the Outer Path, which gives him power over life and death.
Naruto
Naruto is the protagonist of the "Naruto" series and has become one of the most powerful ninja in its history. With almost limitless potential, he has mastered the Kyuubi nine-tailed beast sealed within him, giving him enormous chakra reserves, over and above what he has inherited from his distinguished parents. He makes extensive use of the shadow clone skill, and has mastered the powerful Rasengan technique. He is able to summon the frog boss Gamabunta, and has begun to exhibit the lightning fast taijutsu skills for which his father was known. During the course of the series, Naruto has defeated powerful foes, including Gaara, Kimimaru, and Kakuzu.
Itachi
Uchiha Itachi is the older brother of Sasuke and was a member of Akatsuki. Paired with Kisame, he possessed the powerful Mangekyou Sharingan technique which enabled him to hypnotize his opponents and bring them into an illusory world where he could torture them as he pleases. He can also summon Amataseru, a black fire which can burn anything it touches, and Susanoo, an invincible demonic avatar. Such was his power that even Orochimaru and Madara were weary of confronting him directly.
Sasuke
Sasuke is Naruto's rival and former comrade, who becomes one of the series' primary antagonists. Like his brother Itachi, Sasuke has activated the Mangekyou Sharingan and can use many of the same techniques, including a black version of Itachi's Susanoo. He has defeated powerful foes, including a weakened Orochimaru, Deidara and a weakened Itachi.
Orochimaru
Orochimaru served as the primary antagonist of the first half of the "Naruto" series. A former comrade of Jiraiya and Tsunade, and the student of the Third Hokage, Orochimaru was a member of the Three Sanin and wanted to acquire all the ninjutsu in the world. He possessed a vast inventory of abilities, including summoning the Death God to defeat the Third Hokage and using Impure World Resurrection, which allowed him to resurrect dead heroes and bend them to his nefarious will.
Jiraiya
Jiraiya was a member of the Three Sanin, making him one of the most powerful ninja in the "Naruto" universe. He was also the mentor of the fourth Hokage. Jiraiya fought by turning his hair into needles, like a porcupine, growing and shrinking his hair at will. He also mastered the Rasengan technique, which he passed onto Naruto. His most powerful fighting style is his Sage Mode, in which he fuses the elder toad couple Shima and Fukasaku to his body, and is able to unleash more powerful senjutsu techniques. Even the Pain stated that if Jiraiya had known his secret, he may not have been able to defeat Jiraiya.
Tsunade
Tsunade is the Fifth Hokage and is considered the most powerful female kunoichi in the "Naruto" universe. She is an extremely skilled medical ninja, able to heal even the most life-threatening wounds. While her senjutsu skills are not as refined as Orochimaru's, she possesses incredible physical strength and can summon the slug, Katsuya.