Avatar: The Last Airbender is one of the best shows of all-time, for sure. Season one can be a little weak at times, feeling the most standard and episodic of the seasons though there are definitely episodes that show it's greatness (The Storm jumps to mind). Season one is still good but it's not until Season two when Avatar truly becomes great. I have small issues with how the finale played out, but season three on a whole is fantastic and the finale is still great for the most part. A:LAB has some of the best world-building out there, and it's a genuine journey filled with plenty of interesting locations, and most importantly, fantastic character arcs. Zuko's character arc is something I still see being praised all these years later, rather it be on twitter or AskReddit threads about greatest characters, and it's well worth earned praised. Though everyone really has their own growth, and I like that the series while has segmented story arcs (Ba Sing Se being the best) is one truly realized narrative that was clearly well thought out. Also, the action choreography is top-notch, the best western animation has to offer, and among the best counting anime too. My favourite characters being Sokka, Toph, and Zuko (roughly in that order), with my favourite episodes being The Boiling Rock two parter, Tales from Ba Sing Se, & Zuko Alone (again, roughly in order).
As for Korra, I loved the first season . . . until the last two episodes. The halfway point of the season features the most dramatically tense sequence in all of Avatar, legitimately edge of my seat content and it developed such an interesting conflict throughout the season. I don't want to get into it do to spoilers, but there's not many shows that nosedive in quality for me as hard as Korra did in Season One, the last two episodes basically killed the show for me. There was one scene in the finale I really liked, honestly shocked Nick allowed them to air that one tbh but it was handled really well. Continued to watch a bit of season two but I didn't like it, ended up giving up before The Beginnings two parter which was the only reason I was giving season two a chance. I'll probably watch those at least at some point, I hear they're pretty good.
And in fairness, I've heard great things about the last two seasons of Korra, and from people who agree with me about season one. So I should probably give it another chance.