Drayden is the best. Gym leaders are generally the pillars of their community, and Drayden happens to also be the mayor of a major city. He is a wise man, well-versed in his region's history. Drayden is especially heroic to me because he not only raced to N's castle to protect you from the sages alongside the other gym leaders, but also drove Team Plasma out of his city and helped save everyone from nuclear winter.
Of all the candidates we've seen, and there have been some good ones, I think he is the one who most deserves to win.
Alderr I think would be good too. As champion he is already a type of fatherly figure to his region fact. His warm, sunny disposition and life experience would make him a better leader perhaps than trainer. He was defeated by Team Plasma, but still acted honorably. He is great with inspiring kids and reaches out to people through a school. So he is my second choice, its tough having so many good options.
Iris is a little young and immature for job now, but she might grow into a good leader one day. She has a good mentor in Drayden who could prepare her for that role, and she has a lot of enthusiasm about what she does. She's a talented girl in game and the best of the Unova champions to me. For now, I am going to vote for Drayden for the leadership position, but in a couple of years I might be supporting Iris too. Who knows?