Season 3 was my favorite for Avatar, followed by 2. I think the main reason for this was that the season 2 finale and season 3 opener took some of the edge off Aang's childishness, which I found unappealing in the first two seasons, and we finally got Zuko on the right side. I also really liked Aang's costume change in s3; that yellow poncho look never did it for me.
I liked Korra a lot, too, with most of my problems with it being easily attributed to having to cram too much into too few episodes. However, I do kind of wish that there had been a plot twist when Mr. Sato revealed himself as an Equalist, and while Asami still rejected him,
Mako joined him. It still strikes me as odd that they would go to the trouble of establishing that Mako, Sato, and (falsely) Amon all had family taken away from them by firebenders if they weren't going to actually make anything of that connection. It would have been a real twist if Mako came around to believing in the Equalists' ideals because of this link and his first-hand experience with how benders can cheat and destroy. And holy crap, what a victory for the Equalists it would have been if a popular young bending athlete with direct ties to the Avatar herself joined their side, let alone if he
voluntarily gave up his bending.
It would have made Mako's character way more interesting, and would have had the second benefit of letting Korra and Asami's relationship be developed without their love triangle being the main focus, even if it was still simmering on the back burner.