Integrity

The MMM has on more than one occasion focused on the leadership quality of integrity.  Reputations and relationships rise and fall based on integrity or the lack of it.  I try to live a life where integrity is not only important to me, but is also clearly visible to others.  I think it is one of the greatest keys to leadership, not to mention one of the finest qualities we can pass on as parents.

In looking for people to hire, you look for three qualities:
integrity, intelligence, and energy. And if they don't have
the first, the other two will kill you.
--Warren Buffet

Warren Buffet sums it up nicely but I would not limit it to employees.  I look for integrity in people that I develop relationships with in all aspects of my life.  So many great qualities are found where there is a solid base of integrity.