This is my Avatar tierlist based on how good of an avatar I think they were. Be warned I don't know much about Avatar Kuruk, Kyoshi and only a little about Yangchen from the comics.

https://preview.redd.it/b3nzgdqbnqbd1.png?width=1140&format=png&auto=webp&s=18dadfc5bbf01b714b7d9b2fc0ef8c4e3e7ca486

Wan - He essentially pioneered the whole thing, being the first to use all 4 elements, commune with spirits and reclaim the world back from the spirits on top of beating the most powerful evil entity when bending was barely a thing.

Korra - She's encountered great foes who have threatened to cause chaos and disrupt the balance of the world, while bringing in positive change. Considering she's done all this in the beginning of her journey means there's a lot for her to accomplish.

Aang - While he has some decent accomplishments, a lot of what we saw in the show and the little I know about the comics, other characters tended to lead the path more often than he did. I think Aang is the most forgettable character in the Gaang, and even though he beat the Firelord, all of the changes that happened during and after could have been done without him.

Yangchen - I don't know much about her other than she fought a spirit or something, but she seems solid.

Roku - He's so far the most 'normal' avatar, from what I know he did a bunch of mediation among smaller conflicts, so that's fine. But this guy proceeds to beef with a volcano, die, and his friend who he spared ends up causing the worst war in human history. I can't blame him for that though.

Kuruk - Bro died fighting for his girl and still lost her, that's his biggest accomplishment.

Kyoshi - I like her vibe but don't think she was a good avatar. She hogged the the Avatar role for over 2 centuries and in that time, her biggest accomplishments are killing Chin the Conqueror, who she killed by ACCIDENT and the only reason she dealt with him was cause he bothered her in her home town, like are you serious, he had conquered the entire earth kingdom outside of Ba Sing Se. She also formed the Dai Li, the most corrupt government agency in the Avatar Universe. The only reason she's above Kuruk is cause she looks cool as hell.