This really depends on how mass producible Piltover and Zaun’s equipment are.
Frankly, Cait’s equipment is roughly WWII era tech. And just about everyone else in the city is much more advanced.
Even if they can only produce smoothbore muskets creating Napoleonic Era infantry lines should crush any army using Renaissance tech or lower: Demacia, Noxus, Targon, Shurima, Freljord, I mean basically everyone else except maybe Bilgewater.
Provided the commander is not an idiot to get themselves surrounded and trapped by the enemy. We actually have data for this, during the great European Imperial Expansion it was not uncommon for the European powers to fight and win battles where they were outnumbered 10 or 20 to 1.
And again, this is pre-Napoleonic Era, I’m using as a basis. Despite the fact no one in Piltover is based on pre-Napoleonic tech. They’re all 100 years more advanced or more.
Now, the only possible counters to this are the ridiculously strong characters that every setting has been getting for the last couple years.
Sion is now strong enough to push 300 tons or something. Nasus et al. are no longer somewhat strong librarians they’re basically demigods. As are the actual demigods in Freljord.
I don’t know what would happen to these characters should they meet Piltover tech. Apparently Aatrox has killed demigods. Can he do that because he is basically a demigod himself? Or is it just a level of strength that he has, which in theory everyone can do?
If the latter, the answer to dealing with all these creatures becomes “five rounds, rapid” or “I love the smell of napalm in the morning.”
If it’s the former. Then this whole thought experiment is pointless, as the answer is “whichever side has more demigods.” Of which I think Void or Targon wins. Targon can beat almost anyone if they could get Zoe to actually use her power efficiently.
Now beyond military there is another factor that determines success in war that often gets overlooked. Government stability. Basically when a war starts to last years, leaders start to die and infrastructure crumbles. Then that starts to have more of a barring on how successful a military power can continue to wage war.
On this front Piltover actually is pretty bad. Since there is already a revolution brewing in Zaun. Now these disgruntled revolutionists can be squashed through early military victories creating a pro-Piltover wave of nationalism. But if their military leaders are incompetent I can see their government crumbling.
Noxus is terrible in this regard. We’ve already seen this happen in the lore. Things take a bad turn in Ionia and the government collapses in on itself and they abandon the conquest. Swain may be a genius but his government is new and as far as I can tell doesn’t have a succession plan. If things start to go bad they’ll get knocked out of the fight.
Same is said for Freljord. They already have a history of cyclical violence against each other. Should they start to lose, the three tribes will crumble and lose any unity they may have had at the start of the conflict.
I have no idea what even is the ruling bodies throughout Ionia and Shurima so I’ll just skip them. I know Azir is trying to create an empire again. But I don’t know what governments he’s replacing. And if he has to create an entire empires political systems and infrastructure from scratch. If he is, well, good luck mate. But I’m betting against that succeeding.
Which brings the surprising winner on that front to be Demacia of all things. Medieval peasant revolts are easy to crush. It has a well defined succession system with an heir already ready to go should J3 die.
If Demacia can last the storm, they may be able to pull the surprise stability win. But since I place their military very low on their chance to defend themselves I don’t think they’ll make it that far. They’re a backwards nation that doesn’t utilize its mages and has little in the way of tech. With an isolationist attitude that will only hurt them in gathering allies and tech.
or maybe even 
...
They always have a greater plan I feel like*[leblanc-funny]*
