HA, nobody has voted for Book 2 yet. VICTORY! I bet this is how Book 2 feels:
Well, my take on this is Amon was good until his character development dropped the ball. The overfocus on bad romance plots and unnecessary pro-bending arcs would have been better spent bringing depth to the legitimacy of the bender/non-bender conflict. If the romance plot was taken out or, at least, cut in half and the pro-bending was also cut in half (better written would have helped too) for the sake of a more complex overall plot, Book 1 might overtake Book 3 for me.
Book 2's main problem was mostly the continuation of the romance a bit and the obvious telegraphing of Unalaq as the villain. Korra dumping Tenzin the way she did was pretty stupid too. Ignoring Wan, Book 1 was better minus the probably even more unsatisfying, incredibly Deus Ex Machina ending. I'm not sure since there's 2 Deus Ex Machinas in Book 2 versus the 1 in Book 1 (unless you count Korra unlocking airbending through the power of friendship), Jinora Ex Machina and Giant Spirra Korra Ex Machina. Kind of gives Book 2 an unfair advantage and yet I still like its ending better! Korra had good moments, but remained decent to me here.
Book 3 was my 'This is good overall' moment. We always had a sense of what these guys were. The only big reveal was that they were more cult like than we initially thought and hence insane. What made Book 3 good really besides the relatively unique villains, in the superpower department at least, was that the main characters were as likable as I wanted them to be and reached where they needed to be in terms of activism and getting their moments. Nobody really got the short end of the stick in this regard. I really grew to like Korra here because she was still her, yet also more mature and calm. I wish the series had started out with her the way Book 3 starts out with her. Sigh, c'est la vie. The fact that the main villains don't drop the ball as hard as Amon is definitely icing on the cake. I was a little disappointed at the political strawman though.
To sum it all up Book 1 had wasted potential, never delving into the oppression themes then really took a dive with that ending. Book 2 was just worse in every way. Korra learns nothing, the villain is very obvious from the start in a bad way (I say in a bad way because Kuvira is currently obvious, but I don't mind the way she is obvious), annoying romance still rears its head, Jinora Ex Machina, Giant Spirit Korra Machina. The only good parts were Wan's story, the spirit world, and the side plots. Book 3 fixes everything with romance side plots that are tolerable and with characters that won't make them annoying and just a better, more personal story. While obvious, I liked how Bolin's metalbending training was just a bait and switch to hit us with him lavabending. Book 3 lacks the impact of Amon, but makes up for it by ending more competently and never really hurting the legitimacy of the villains despite them clearly acting insane.