This is a hard question to answer, and my personal opinion is based on my experience in the Army over a while. The best leaders I encountered managed to somehow

This is a hard question to answer, and my personal opinion is based on my experience in the Army over a while. The best leaders I encountered managed to somehow turn out the best in the people they led. That can manifest in a lot of ways. Improvements on subordinate performance, increases in technical proficiencies, a more disciplined approach to their work. Those are all good metrics, but the best leaders managed to get their subordinates to actually want to improve on their own, without sufficient goading from their leaders. Most of that, therefore, lands in the realm of understanding group dynamics, behavioral economics, and leadership psychology.