These fights are happening where the season 3 finale took place.
Aang (not poisoned): I don't think it's controversial to say that Aang wins this one. Zaheer was barely able to keep up with Tenzin, and Aang is much like Tenzin, but with 4 elements. Honestly, Aang would just need to go into the Avatar State, form an element ball, chase down Zaheer, and take his bending away. It's honestly kind of an easy fight for Aang.
Aang (poisoned): Aang is tapping out of this fight pretty quickly. I'm not trying to say that Aang is physically weak or has low endurance and durability. But, compared to Korra, he definitely has less. Aang also has the grave misfortune of not being able to breathe fire, thus not stopping Ming Hua's ice spear.
Then again, there is a chance that a good airblast from his mouth would dislodge its direction. Anyway, it then would come down to will he be strong enough to break through the platinum chains. If so, he'll probably get out of the cave. But, he's not going to hold out much longer. I also think there is an argument for him not being able to energybend due to his energy being weak, but I'm not going to make that point since it's kind of irrelevant. Anyway, Aang is dying if he's poisoned.
Yangchen (not poisoned): Similarly to Aang, if she's not poisoned, she's beating Zaheer. It's be the same scenario as Aang, just without the energybending. It's be deathbending.
Yangchen (poisoned): She's not even getting out of the cave. Physically, she is much weaker then most of the Avatars. She may have endurance and is durable, but not to the point where she is beating the poison.
Wan (not poisoned): Much like most villains the later Avatars face, Wan is at the disadvantage of having outdated bending. Again, his only special thing is the cloud, and it's not known if that cloud is as fast as Zaheer's flight. And if it was, what could Wan even do? In theory, he could go to that pool that Korra goes in and try to hit Zaheer with long range water strikes. In his fight with Vaatu, it seems that he's good at aiming. Honestly, Wan wouldn't hold out too long in this fight.
Wan (poisoned): Wan isn't winning this, but I actually want to ask everyone's opinion on this: would Wan have the strength, endurance, and durability to get out of the chains and out of the cave? Because Wan was able to barely get injured when fighting the carpet/kite of evil.
Kyoshi (not poisoned): This would be an interesting fight. Kyoshi is great with earthbending, and with the pillars and mountains around her, she'd have a field day throwing them at Zaheer. Problem is that Zaheer is really fast. Honestly this fight either ends with Kyoshi suffocated or Zaheer smashed to pieces. I assume Zaheer smashed to pieces is more likely.
Kyoshi (poisoned): Kyoshi has the benefit of being tall, strong, durable, and endurant. She'd break out of those chains, start lavabending like a mad woman, and destroy the cave. If Zaheer escaped that, she's giving him a chase, but probably not the greatest one. Anyway, he's getting smashed to pieces or burnt to death from a pillar of lava.
Roku (not poisoned): I'd give Roku the win. Just look at this un-Avatar Stated fireball:
I don't know how well Zaheer will dodge multiple of that attack. Also, like Kyoshi, Roku can just lavabend pillars at him. Zaheer loses.
Roku (poisoned): Yes, I understand that Roku was old. Yes, I understand that poison gas is not the same as mercury. But, if that is any indication of his ability to withstand toxicity, then he's not getting out of that cave (is the gold standard just getting out of the cave by this point?). If he were to withstand that toxicity though, it'd be much like a Kyoshi fight. Anyway, with the information we have now, I think Roku loses.
Kuruk (not poisoned): Kuruk is a hunter. I think he'll be able to hunt his prey in the sky. Kuruk is winning this one. Again, throw enough elements without poison in the system seems to work. Also, if he gets to Zaheer, he'll just one punch him.
Kuruk (poisoned): Kuruk's getting out of that cave (the hardest part will be the ice blast). He's then punching Zaheer so hard that he dies. Kuruk is a menace.
Anyway, in the end of it all, they all lose if they're poisoned. Without a metalbender on hand, they're all getting it.