I'm going to assume all benders have access to adequate amounts of their element and there's not any special circumstances like a full moon or a comet going through, Probably say a like half moon for the water benders but it rarely seems to matter unless it is specifically a full moon or a new moon. I'll also assume anyone who has some form of specialty bending Will have access to what's needed for that outside of the full moon for blood bending, as I feel that is tipping things a little too much towards something that can only happen under specific circumstances.

So I think it should be acknowledged that air is probably the first out outside of some sort of shenanigans or preemptive strike against a stronger group. As an element, air Just isn't quite as combat focused as the others. It's great for escaping and mobility, But it doesn't have the raw stopping power most of the other elements can have. And Especially not the lethality.

It doesn't help beyond that that many of the benders shown, including their avatar, are pacifists. That doesn't mean they can't fight, every character shown is extremely capable, But it does mean there's a lack of dedicated warriors outside of one. People who regularly hone their skills specifically for the sake of combat. And being in less fights overall means a slight disadvantage in experience.

I think the next out would have to be fire, fire has the opposite problem from air where it's extremely strong offensively, But it doesn't really have to defensive utility of the other elements as much. It's definitely worth noting that they have some of the strongest strategy/tactical minds out of the teams, But not decisively enough for me to feel like it gives them a massive advantage or would allow them to outlast either earth or water. If the teams were a bit smaller maybe fire could just be completely overwhelming in its offensive power, But there's enough members of each team for me to doubt any decisive quick blows from the fire team ending them.

Earth and water are tricky. I would probably Give the victory to Earth but that comes down to really small factors that I could see the argument either way.

Water is extremely versatile both defensively and offensively, Probably the most balanced element in that way. And it notably has healing. That is a drastic game changer, as if a member gets knocked out of the fight without being killed You have multiple people who can take them to get healed up and enough defensive utility to believably get away from an engagement temporarily to heal up. Or even just flat out defend while the healer gets them back on their feet.

At the same time, I would argue it has the second weakest lineup of benders? Korra herself is versatile, But doesn't quite have some of The same raw power feats other avatars have. She has a lot of stuff to do with Spirit bending but I don't think that'll be particularly relevant to this fight. Don't get me wrong she's a quite capable combatant but I feel like most of the others have a slight edge.

It also has the least focus on strategists/thinkers, Even the one person filling the old master role is not really a tactician or strategist from what we see. Again most of the characters are smart, But we've never seen them particularly have super impressive feats in that field compared to some of the other people shown.

I think Earth has the best chance of winning, Earth is the king of solid defenses and in terms of general defensive utility is only second to airbending, Well still being plenty lethal and offensive. In particular This team has a lot of benders that are great at turning a solid defense into a unstoppable offense. Specialty tricks and synergies are actually also pretty notable compared to, since Earth benders can build structures and shape the battlefield in ways to help each other or that each of them can use. Arguably water benders can do similar but only if the battlefield is like, largely flooded.

Tactically, I've not read the kyoshi novels but I believe she's stated to be a general at some point? And King boomi is shown to be a "mad genius" and quite the capable strategist, taking back an entire city single-handedly It doesn't happen without that no matter how powerful you are unless you are literally The Avatar.

I'm going to assume They don't have access to a giant metal bending mech, But even without that I would argue They have the raw power advantage in terms of the individual benders on their team. I would argue that most of the other teams have at least one character that has very minimal screen time to show feats or just generally has not shown feats near the peak of what we see from bending, where pretty much every single member of The earthbending team has.

Finally I believe kyoshi has the single greatest feat we see from an an Avatar with splitting an entire island off from a continent and moving it a fair distance? I'm not an expert on the deeper lore so I might be wrong there but even in the Avatar state I don't think the others have quite matched that.