Unlike the last ones where the fight takes place where the OG final fight takes place, I think this fight has to be outside of Zaofu to be fair. The inside of the Colossus is a no go for all of them. Because none of them can get in.

Aang: Aang beats Kuvira, but I'd say only this setting. Because if there was enough metal to form a box, Aang isn't escaping that (what is it with firebenders having trouble melting metal?). Anyway, in this situation Aang beats Kuvira. Kuvira is very fast. But, Aang is faster. Without the Avatar State he can toss her around with air. Aang would only be in trouble if she were to pin him in the air with metal. Without metalbending Aang is kind of screwed. But, if he was in such a situation, he could try to enter the Avatar State and try some type of air blast that could distract Kuvira causing him to drop to the ground. But, I feel like Aang getting all metaled up is unlikely, because I repeat, Aang is as fast as the wind. Anyway, Aang traps Kuvira in earth, and would have to immediately take her bending so she doesn't break out of her earthen confines.

Yangchen: Yangchen's fight would be faster. Like Aang, she'd start off with some hard air gusts, and move faster then Kuvira (isn't Yangchen faster then Aang?). If the fight gets difficult for some reason, Yangchen could use her scream and completely knock out Kuvira's ear drums, hurting her long enough to suffocate her or crush her with a rock.

Kyoshi: This fight is harder then people think. Kyoshi wears a lot of metal. Her fans, her chain mail, and her head dress are all metal. This leaves her in a terrible situation. It also doesn't help that Kyoshi's big and kind of slow. If Kuvira acts very quickly, she could just lift Kyoshi in the air and try to suffocate her or hurt her with all the metal she's wearing. This would definitely suck for Kyoshi, but I doubt that Kuvira will be able to kill her this way. Kyoshi can breathe fire, and she could just go into the Avatar State and shoot out a flame with her mouth, hopefully distracting Kuvira, or burning her. It all depends on how far Kuvira is. Kyoshi would have to lose basically all her armor, which really sucks though. Anyway, an opponent of this magnitude would be immediately on Kyoshi's have to kill right now to do list. Without her fans, airbending and earthbending on a smaller scale is going to be hard. But, with what she has, she can still try to make the entire area a hell scape. She could either a) create a giant chasm and with all her might crush Kuvira in it, b) lavabend everything, or c) hopefully get close enough to Kuvira to freeze her internal organs. Anyway, option a) is the most likely choice, and Kyoshi's power (especially if entered into the Avatar State) would crush Kuvira. Or burn Kuvira. Or freeze Kuvira. Kuvira loses.

Wan: Wan would have the best chance winning against this villain then the other 4 I did. It is unclear the range of Kuvira's metalbending, but if Wan gets onto his cloud and gets up there ASAP, he might be able to dodge it. In Beginnings, we see that Wan can send sizable blasts of fire from the sky as well as well as bend the earth bellow him. In theory, he could glow it up, and shoot some of that fire at Kuvira. Kuvira would either a) move, b) push rock over her to protect her, or c) burn to a crisp. If Kuvira survives, she can't do much but throw rocks at Wan. He can evade or catch them and throw them back. Kuvira could try the dicey move of pushing a column of earth into the sky for her to stand on. The only problem is that if the action isn't done fast enough, Wan can aim a good shot of fire. Anyway, Kuvira would either get into the air and kick Wan off his cloud or aim so well from the ground that he falls off. Anyway, a good metal jab, and he's dead.

Kuruk: What is Kuvira going to throw that's going to stop Kuruk? The metal would probably hold him in place for a little bit. The Avatar State is great. Anyway, he'd knock Kuvira around, always getting up when getting hit, and then a good punch will kill her. And if you're thinking I'm giving Kuruk too much credit, please re-read his chapter in The Shadow of Kyoshi.
Roku: Kuvira would probably try the good old strangle him with some metal technique. Roku's head piece is medal, and she could probably try to strangle him with it. I doubt it will work, but she could try. Honestly, with all the power Roku has when not trying to fight a volcano, he could take down Kuvira easily.
As I think about these fights more in depth, I feel like I start to realize how powerful and weak characters actually are in different situations.