Socrates is often considered the wisest man because of his willingness to question everything. The inventor of the Socratic method, Socrates believed in examining ideas or thoughts through additional questioning of them. This would help stimulate critical thinking and allow others (and Socrates himself) not to take certain ideas for granted or to accept them without thinking properly.
Socrates is also considered the wisest man because of his humility. He was not arrogant about himself. He claimed that he was wise because he was able to admit to himself that he did not know everything there was to know about life. As a result, he did not take whatever knowledge he had as absolute and final. There was still room to question what he assumed that he knew.
Lastly, Socrates is considered wise because of his teachings regarding worldly pleasures and the need for self-discipline. While he did not appear to be an ascetic, Socrates taught that money, fame, or sensual pleasure could often imprison those who indulged in them. The desire to not lose these things might prevent someone from living freely or taking the right course of action. Addiction to such things could also lead to painful complications for the individual.