Itachi is the best one to me. Not only because he was powerful as f*ck, but because of his personality, good heart and selflessness too. Even being one of the most powerful characters, Itachi never was arrogant, he was always kind, gentle (specially with Sasuke before the Uchiha massacre) and very clever too. He chose to live in the shadows and to be renegade forever, suffering alone, in order to achieve a greater good (Konoha's people safety) and to protect his little brother. Not to mention the fact that he never really enjoyed fighting, that's why we never saw his full potential.