What is the single greatest "trait" you look for in a Leader?

Empathy

A trait I think is important is integrity.

There are three characteristics of a leader. Every leader must have them. Hunger, honor and honability. Without any one of these you won't have a good leader. Without hunger, there's no drive. Without honor, there's no trust. Without honability, they'll never learn.

Never stop never quit

Mutual respect.

The strikingly handsome face of a Ranger.

Integrity.

For me, personally, the most important trait in any leader is accountability, which includes the ability to own a mistake, to take the blame when due and push forward. Mistakes happen, especially in a military environment. Training alleviates the majority, but we are human, not machines. Humans make mistakes; the mark of a true leader is the ability to face said mistake, and to learn from it and continue to grow. Once you take the mantle of leadership, from team leader to General, doesn't mean you stop learning...

Common sense

If I had To pick someone from one trait only it would be compassion. A compassionate leader will lead from the front, will earn the trust and respect of his subordinates, and will understand the importance of his/her role in a leadership position. If the person is considered for a leadership position they likely already have the experience and knowledge required. Second to compassion word be confidence. These two things aren't easily learned. They are natural traits that exist in natural leaders. They can be learned, but not as easily as div-Pam could be memorized cover to cover.