This is WAR!!! What region do you want the help of?

Genuinely tough choice

I mean, Sinnoh has Cynthia as well as the dieties of Time, Space, Chaos, Creation, Nightmares, Knowledge, Emotion, Willpower, as well as more

Very valuable if I can get their support

But then let's look at Kanto

Nothing strong or worth noting there in terms of people, but Mewtwo lives there

And also, if I'm willing to get a bit Shady there Actually is a person

Giovanni

Him with Team Rocket could make a great army

But what would truly make Kanto valuable is its ties to Johto and Hoenn

Honestly, not even Johto

Heonn has much value to me (especially in terms of Pokémon)

But by making one alliance I make three that would increase my numbers and support as well as take away what my enemy can get

Would be extremely smart, honestly

But I'm going with Alola

Many powerful trainers live there, the Tapus would also be assisting me, I can use the Ultra Beast problem to by advantage, and Sogaleo, Lunala, and Necrozma are a pretty powerful trio

I know I mentioned powerful trainers, but I'd like to point out that Red is there along with Blue, and if I can get an alliance with them I might be able to follow up with Kanto (the place they come from) and perform the plan above

But if not I still at least have them two

Alola is just the strongest choice

I would like to give a shout out to Kalos, though

Mega Evolution and powerful Legendaries are a good choice

Plus, they have experience with war