Both series. I can not decide me between LOK and ATLA. Sure, both series have good and bad sites. But if one of them didn't exist, the other wouldn't be so good, I think. Both series are the best of the world.
Growing up with atla I would have to choose it over korra . Really though there both amazing. @ Aang 112 that picture ur using is awesome, kind of wish that's how korra ended with korra and all her past lives drinking tea telling stories in the spirit world.
I also prefer ATLA. There were many things i didn't like in LoK, mostly because i keep comparing to ATLA. LoK is good as a show but i think they changed a lot of things if you compare what was said in the previous one.
EDIT: I don't know who is the white haired girl behind them in Aang 112' picture. Who is she?