This one is definitely easier to answer! There are a couple regions where two kids could work as the protagonist based on when we are in timeline, but otherwise there’s a pretty distinct main character for each.

Kanto – Sage
Johto – Renato/Celeste
Hoenn – Levi
Sinnoh – Tanith
Unova – Euphie/Maddie
Kalos – Aurélie
Alola – Primrose