Top20 - Top11
20. Orochimaru (Naruto)
19. Arata Kasuga (Trinity Seven)
18. Koro Sensei (Assassination Classroom)
17. Gilgamesh (Fate/Zero)
16. Erza (Fairy Tail)
15. Haruhi Suzumiya (Melancholy of Haurhi Suzumiya)
14. Alucard (Hellsing)
13. Zeref (Fairy Tail)
12. Levi (Attack on Titan)
11. Kumagawa Misogi (Medaka Box)
Top10 - Top6
10. Natsu Dragneel (Fairy Tail)
9. Kaneki Ken (Tokyo Ghoul)
8. Kirito (Kirigaya Kazuto) (Sword Art Online)
7. Meruem (Hunter x Hunter)
6. Sakamaki Izayoi (Mondaijitachi ga Isekai kara Kuru sou desu yo?)
Top5 - Top1
5. Madara Uchiha (Naruto)
4. Ainz Ooal Goan/Momonga (Overlord)
3. Accelerator (A Certain magical Index)
2. Goku (Dragon Ball Z)
1. Tatsuya Shiba (Mahouka no Rettousei)
Jotaro Kujo: JoJo's Bizarre Adventure
Star Platinum's many different abilities including extremely high strength and agility as well as heightened senses and the ability to stop time make Jotaro's stand nigh unbeatable.
Akashiya Moka: Rosario + Vampire
A Vampire who can't die unless you are a top rank demon yourself what else ?
Issei Hyoudou: High School DxD
Who wouldn't like a pervert that can double his own power, Turns into a dragon lord, has in Army of girls and is Badass.
Kojou Akatsuki: Strike the Blood
He's the most powerful vampire to have ever lived as he is the fourth progenitor. What more evidence could you want?
Sachiko Shinozaki: Corpse Party: Tortured Souls
She's a psychotic killer ghost, with no sense of mercy or pity. With enough power to be able to create her own parallel dimension specifically for the kidnapping, torturing, and killing of innocent people.
Seijuro Hiko: Rurouni Kenshin
Taught Kenshin and, even at Kenshin's best Seijuro Hiko can kick his butt.
Whis: Dragon Ball Z
Whis is able reverse time with his own body, and is also capable of destroying an entire universe. Beerus, the God that Whis watches over, can destroy a planet within microseconds effortlessly. We find out that Whis is on another level when compared to Beerus, making him the most strongest character in Dragon Ball Z yet.
Gasai Yuno: Mirai Nikki
No powers = no problem
Takeo Gouda: Ore Monogatari
Sadao Maou: Hataraku Maou-sama
he became a good DEMON/DarkLord
Sebastian Michaelis: Kuroshitsuji (Black Butler)
A powerful demon, working as a butler to help with revenge. I really appreciated that he and Ciel were close, without Sebastian having to question his demonicness or anything.
Kirei Kotomine: Fate/Zero
One of the best anime villains, essentially becomes immortal at the end of the series. Also one of the most smartest and cruel players in the Holy Grail War.
Ayaka Kagari: Witch Craft Works
She has the power of satan, literally.\nShe can control fire in a way that not even Natsu Dragneel can (no hate)
Even though he always ends up terribly wounded, he just goes on like it's nothing a few minutes later in a few scenes
Sora and Shiro: No Game No Life
He may not have any magic abilities or spells or superhuman powers, but that doesn't matter when you have the brain of a supercomputer, his skills at analyzing a situation could easily get himself a win, even against the toughest opponents!
Trafalgar Law: One Piece
He can control everything in his "Room"