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

One who leads by example, willing to get their hands dirty earns my respect. Shows that they too are part of the team. Goes back to the old infantry call "follow me"! Out of the trenches of WWI. Puts the "lead in leadership".

Can't look for just one as one trait doesn't make a good leader

Integrity

Someone who gets out and works with their team/squad.

That they lead by example and not by word.

If I had to single out a trait I would say maturity.

For me, I think it is a "don't forget where you came from" approach. A lot if "leaders" I had to deal with my last few years in made rank and immediately became Bullies. They forgot what is was like to come through the ranks

Patience

Humility is one of those keystone traits which translates into a host of others.

The building block is INTEGRITY.