Favorite in ATLA: Jet. DON'T JUDGE ME. Favorites in LoK: Meelo, Mako, Tenzin, and General Iroh. From the first series, as much as I love the main characters, Jet just did it for me; I loved his complex character/story. As for the second series, I really enjoy the silliness/humor provided by certain characters such as Meelo and sometimes Tenzinand others - again complex personality/story which are Mako and some what General Iroh. 3nodding
My very favorite character from the first series is Zuko, he takes my breath away along with his end of the series sportsmanship and loyalty, and my favorite for the new series is Mako, yes they are both troubled (and hot) firebenders. i can't help it.
out of both series though, Uncle Iroh is my favorite, he shows courage and change, he connects with his peace and spirituality and understands the complexities of all of those around him, especially his nephew and his sister.