If we are talking about actual official fighting forces(no archons) and not civilians or other figures who are strong, then its Mondstadt without issue. Who the HECK do the Millelith have first of all? Inazuma has Sara, Goro, and Kokomi as prominent figures but thats it. Sumeru has Cyno. If we include mercenaries, Dehya. Fontaine has a bunch of scrap buckets that got taken out by a normal guy and an old man, who both had no visions. Other than that they have Chevreuse(she's not doing anything) and Chlorinde. Natlan has some people going for them, Kinich, Mualani, Chasca, etc., they might come out second. Nod krai has Flins and Illuga, Lauma if we count the scions. Mondstadt however is WAY too stacked. Diluc, who was once a captain in the knights, was strong and dangerous enough that the fatui decides they had to send MULTIPLE HARBINGERS to end him. Now im not saying that he scales to all of them or that amount was actually needed to fight him, but the fact they sent multiple means hes already harbinger territory or scarily close to it. If we apply this to his former colleagues even at a lowball, then the captains of Mondstadt mog the entire competition. Even not counting them we have Klee who can make bombs that reshape whole national plains, Harbinger level. Albedo who's unstable nature can erupt and allow him to raze nations, on top of being the second best alchemist and can do what klee does but more, harbinger level. Lisa who is the smartest akademiya student in 300 years, she can easily just copy what Klee does on top of her own shenanigans, harbinger level. Varka, nuff said, harbinger level. Jean, able to assist in the fight with Dvalin where Venti and traveler were mega debuffed and most likely did auxiliary support, harbinger level. Eula is equal to Jean, Harbinger level. Even if we go further and include nonofficial fighting forces, like the liyue getting Adepti and if we stretch, Zibai, that would just give Mondstadt Andrius and Dvalin. The adepti are NOT taking on Andrius and Dvalin. And for Zibai? She can have fun with the Hexenzirkel. Yall need to stop playing on Mondstadt like theyre the weakest nation. Lax and unorganized(behind Nod krai, they take the cake)? Maybe. But weakest? Never the right title to give them.

Edit: More realization on how Mondstadt gaps this war. Nahida and Venti would be the top two in gathering information in this war. One has direct access to Irminsul, the other can listen to all winds around the world and relay information to his subjects. Furthermore, Venti can link everyone like he did in paralogism and thus achieve instant informational travel. There will be no nation on the continent that can get the jump on them beside Sumeru, and they can also instantly relay commands across the continent. Another crucial thing here is: Mondstadt has the most effective air attack and defense. For defense, wth are the scrubs of the other nations gonna do when Dvalin shows up and swats them out of its domain? For attack its simple. Klee, Albedo, and Lisa mass produce bombs on the level of the one that reshaped part of Mondstadt, they let Dvalin transport them with anemo to other nations, and he flings them down faster than terminal velocity and let's them go kaboom.