My pick is Lelouch from the anime Code Geass.

what introduction does this badass character need, he play both the hero and the villain, with his alter-ego Zero, he is the leader of the resistance group going againts the Empire and then theres Lelouch, the son of the Emperor and 17th in line as successor to the Empire's throne.

This guy will do anything to find out who killed his mother and made his little sister live in hardship and blind, he will fight his own relatives and friends, backstabbing them, using innocent civilian, control them with Geass, except for when he accidentally cast Geass on his half sister Euphemia, that made her on a rampage and killing innocent civilian. That scene totally broke my heart, how Lelouch is powerless against his own recklessness and watching his beloved sister turned into a mass murderer. All he can do is giving out the order in tears to shoot and kill her, yup even a cold hearted evil mastermind could felt regret and sadness, his action could not been undone so he just go along and took advantage from it.

So after finding out who killed his mother (his own father the current Emperor of the Brittania Empire) and killed him, he decide to succeed the Empire himself and rule with an iron fist. Eliminating and capturing all the throne's heir, then proceed to inflict fear and made the empire to be a force so big that nobody could dare to go against at.

At time time, i'm really puzzled to the sudden personality change in Lelouch, from a lovable character to a power hungry emperor just like his own father who he hates so much. Lelouch then went to a parade that celebrates his achievement as the ruler of the Empire, when suddenly from afar, Zero with a saber stood in the way of the parade. I soon learn from the flashback that Lelouch decide to deceive everybody and act as a tyrant so that the people of the empire and the civilian would stand together and reconcile and end his rule, Lelouch then asked a favor from his childhood and best friend so big that i dont think any of us would accept , and that is to bear the mask of Zero and assassinate him in front of the whole crowd. Ever since then, Zero have been the symbol of peace and unity.

This is why this character is my favorite, even in death he still wishes for everybody well being, can you imagine while you're dying in pain, people around you are cheering for your murderer and you're just laying happy to see people finally uniting. This character is seriously the coolest i had ever seen and i dont believe any character can top him.