I think Daryl's evolution was helped a lot by being separated from his brother Merle; and, being mentored by Rick. I think he finally felt like he was worth something. That someone like Rick would depend on him.
As for Merle. I hated this guy, then last week's episode just totally turned him around in my mind. So, at the end, he gets killed and turns and I'm like --- aw, Merle. Then I realize that I don't really like that character. Michael Rooker just shined in that episode. He got almost a whole episode for himself, and he didn't waste the opportunity.
I’d say Rooker was the best actor on that show.