Only up to the last few episodes of Season 2 and I've gone into detail on Season 1 in the "Rate the last movie/TV series you watched out of 10" thread. Season 1 is a mixed bag due to having interesting characters and world building, but also making the destination of the plot's end too obvious (Aang will learn all the elements and defeat Ozai), as well as it's semi-episodic structure making the quality vastly vary between episodes. We have some good episodes such as the one revealing Aang and Zuko's pasts or the one with Jet, yet just as many terrible episodes such as the one with the two rival tribes or the fortune teller episode that make it obvious it is a kid's show at heart. The Avatar State is also a broken power up that made the season finale pretty underwhelming. Overall, I can't give season 1 any higher than a 5/10.
Season 2 seems to take most of my criticism of Season 1 and subverts expectations. The Avatar State is given some actual drawbacks both physically and psychologically to make it more engaging, all the episodes further the plot in some way and there are great improvements in regard to character development, especially Zuko. I still have a few complaints, such as Mai seeming very one-dimensional, at least compared to the other characters, or Toph's vibration sensing not being a valid excuse as to a little girl being able to beat the world's best Earth Benders in a show that acknowledges and limits powerscaling/power creep, but those are minor nitpicks. Only episode I actively didn't like was the latest one I watched, episode 17, since mind control is a terrible, overused trope when we're not given a reasonable explanation as to how Long Feng was able to do so or what his goals truly were. Hopefully, his story isn't completely abandoned after Team Avatar leave the city. So for, Season 2's a 7/10.
From what I've heard from Kenshi, Season 3 is terrible due to feature an abundance of asspulls, not to mention Ozai being a very one-dimensional evil villain. From what I've heard, the cast gain some good catharsis, so I doubt it'll necessarily be a 1/10 tier arc, but I don't have my hopes up for it.