Jesus Christ.
And hopefully to prevent this from turning into a religious discussion: I am an agnostic. I make no claims that he is or isn't divine. I make no claims that he is or isn't a real person historically rather than just an amalgamation of ideas and stories attributed to a name. For all intents and purposes, barring discovery of time travel, he is a real person historically speaking, and probably the most important Philosopher that has yet lived on this island we call Earth.
One could easily spend hours dissecting the philosophies and ideas attributed to him as well as his profound effect on religion and society. But he's my favorite philosopher for a much more limited reason than that. The birthing of an idea ignored for a long time before it was re-popularized by Count Leo Tolstoy in his Kingdom of God Essays. An idea that I sincerely hope continues to grow in popularity, as it has been, and changes the way we treat our fellow human beings.
Every single non-violent movement every pacifist has stemmed from Jesus' sermon on the mount speech. The idea that the only way to break the cycle of violence is to replace it with one of love. That we should turn the other cheek, to not back down but to not respond in kind to those who would hurt us. The US civil rights movement, the Indian independence movement, the mothers of the Plaza de Mayo, the velvet revolution, the Rosenstrasse protest in the heart of Nazi Berlin; are all heritage from that simple idea.