Oh man I love Avatar. Top 3 shows ever actually. If you think season 1 is good wait until you get to 2. Might be my favorite non-Simpsons season of a show ever. Introduces one of my favorite villains too.
As for Korra, season 1 is good with too much high school drama, but an incredible villain. He doesn't just kick ass, everything he does just feels unholy for lack of a better word. 2 is meh. Plots kind of a mess, villain is boring as hell, but it introduces two great comic relief characters (one voiced by Aubrey Plaza) and has an epic flashback story. 3 is amazing, great villains, great characterization and deep story. 4 is good but feels too much like an afterthought and not like a proper ending.