No question. Amon was cool but his backstory ruined him a bit for me. With these four I wish we could get a whole series focused on just them. They're like the morally gray Gaang!

Their abilities were so cool Also a big reason why 3rd season was my fav

I Would Love To See More About Their Past !!!!

And Ming.

Definitely the most interesting; I love them. :D Amon is cool too though.

Amon got a good half-A-ing, Tarrlok just ended up kind of pitiable, Unalaq got a worse half-A-ing. Kuvira had her whole "authoritarian shadow archetype of Book 1 Korra" deal, and I approve...but the Red Lotus were just effectively this supervillain team (or maybe superantivillain team) that kinda got squandered. (I say this as someone who spent most of Book 3 wanting to backhand Zaheer, and the rest of the series wanting to backhand him even harder for being the sole survivor.)

We need backstory comics on those jerkbenders. Bonus points if said backstory comics can convince me to give half a damn about Zaheer outside of a P'heer context; I never had much emotional investment in him, unless "want to smack into next season" (funny how that turned out) counts, for his own sake. After all, I give props to fanfics that manage that.