Before I asked who the worst character was. Now I'm wondering who everyone thinks has had the best development as a character, not necessarily meaning becoming a better person.

While obviously David is up there, I would say Gary is the best developed and probably one of the better and most human characters in the show. He's a good guy with a kind heart, something that was shown even in his earlier appearances when he was treated like a thug. He is clearly flawed and has to live with the consequences of his actions for the most part. He has been built up as one of the more believable characters.

This nearly turned into a full on Gary appreciation post.

TL;DR - Gary has had the best character development on this show, in terms of starting off somewhat as a villain to what he has become, even with all of his flaws due to the show not forgetting them.