Daryl Dixon is a fan favorite "Walking Dead" character, which makes him a prime choice for being the leader of Rick's group in season 6. More important than that, he's also a physically strong individual that cares about the well-being of his party members.
Remember that scene in season 5 when Daryl risked his life to keep Aaron safe? He's been doing that kind of stuff from the beginning, and it makes him a great leader candidate.
He almost got killed in Season 2 in search of Sophia. He tried very hard to find her. He got arrow in his body. Andrea almost shot him in his head.
Daryl always stand side by side with Rick. Rick feels strong with him. Actually whole group feels little strong with him.
On the flip-side, Daryl also has a tendency to distance himself from the group. Especially since the death of Beth, Daryl has become increasingly withdrawn. His fractured mentality might even make him too reckless for his own good. His recent pairing with Aaron puts Daryl on the right track.