Season 1 was an okay introduction to Republic City and the World's general issues, but needed more time and tripped up with it's villain, whose origins were ultimately disappointing compared to the fan interpretations.
Season 2 was fucking terrible except for the half decent episodes that made up the Wan Arc explaining the Avatar's origins which was ultimately disappointing. It abandoned "Balance" for blatant good and evil and the final villain was absolute shite.
Season 3 is widely considered the Best. Red Lotus are pretty good villains with interesting background and powers and they do some pretty cool stuff. I'd put it above Season 1 of the original Avatar.
Season 4 again has a decent villain but this time the series is brought down by it's protagonists who are just straight in the wrong like half the time. Series ends on a kind of half assed note where Kuvira gives up for no reason, the Earth Kingdom is abandoned to balkanization, while the Avatar goes on a Lesbian date in one of the most dangerous places in existence because ????, but all of this is portrayed as the best possible outcome.
All in all the series release rate was unsure and sloppy which ultimately resulted in an unfocused direction in terms of themes, stories and villains. It, like many other series, would have benefited massively from the precise and overarching endgoal of a story that the original series had.