I don't know. How obsessed are you?
I watched it and really liked it. But I only watched it once.
All the characters had such great development throughout the series that it's hard to pick just one. I really liked Iroh, because they first show you a kindly old man, then you slowly get to see why he is like he is, and at the end, he shows you what he's capable of.
The character development was well done across the board. All the main characters learned and grew. At the same time, they all made mistakes and had to atone for them, so no one was perfect. Throughout everything, Aang retained a boyish love of life and a desire to be kind, even to his enemies.
I don't want to list any abilities that would be spoilers, but I'd probably go with water - it seems the most versatile.
I watched
Avatar this last summer with my son (he watched it and insisted I did, so it became a little daily together-time activity for us). I also watched
Korra, but it wasn't as engrossing as
Avatar. I think that if
Korra wasn't a sequel to
Avatar (and so wasn't compared to
Avatar), it would have stood on its own as a great series. I did grin like an idiot every time an
Avatar character made an appearance in
Korra.