My favorite quote on leadership is actually from Nelson Mandela.
It is better to lead from behind and to put others in front, especially when you celebrate victory when nice things occur. You take the front line when there is danger. Then people will appreciate your leadership. (Nelson Mandela)
What I appreciate best about this quote is that it shows how leadership is about building people up, so that all boats rise together. The best leaders are encouraging, but allow people to make good choices and give them the tools to do what needs to be done. Leading is about delegating. Yet when things are tough, the leader has to stand up for everyone. That is when a leader is really needed.
I believe that leadership can be defined in many different ways. That said, different people will have their own definition of what leadership means. For me, leadership is defined through one's ability to recognize a problem and the possession of the determination to fix it.
Outside of that, upon examining different quotes on leadership, the one I found the most inspiring is taken from Max DePree:
"The first responsibility of a leader is to define reality. The last is to say thank you. In between, the leader is a servant."
I have also linked Forbes top 100 quotes on leadership for you.