Definitely Smart.
We have our lead by example stars like Horford and AB.
Then we have our fit the system perfectly players like KO, Crowder, and even Amir/JJ.
But Smart is a vocal leader, an emotional leader, and the type to put the team on his back - mostly defensively, but he sure kills it late in close games. In only his third year, Smart exudes captain - only IT and better shooting stand in his way.
On another note, we have a REALLY likable team!