It's hard to gauge. In terms of just lore, I guess Galar since they're totally battle obsessed, compared to a region like Kalos where battling takes a backseat to fashion to the point where the Champion is also an actress.
In terms of gameplay, we'd have to measure all the NPC's stats and their strongest teams and weigh them against one another, but chances are due to powercreep, they probably get stronger the further down you go, unsure about this one though.
This is also assuming we aren't counting the player character since there's no canon team for any of them minus Red.
If we mix lore and gameplay it probably looks something like...
Galar > Unova = Paldea > Hoenn > Sinnoh = Kanto > Alola > Kalos > Johto
Galar has a ton of insane trainers plus LMAO CHARIZARD IS THE STRONGEST, Unova also has plenty of really strong trainers and feels the most battle centric region after Galar. It ties with Paldea because even if Geeta sucks, Nemona and all of the school staff make up for it, I wonder how many champ tier trainers there are past her?
After that you get to all the mid tier regions, honestly Hoenn/Sinnoh/Kanto all feel about tied. People hype up Cynthia but forget that she's literally the only notable trainer there.
Imo Alola could be higher since the region is crawling with strong dudes, but they only just got their League set up which drags them down, Kukui is an S tier champion btw.
In the lower tier I think Kalos fits, since as I said earlier that region seems way less into battling as a whole, on top of having a very weak cast of rival NPCs. Mega Evolution is the only thing keeping it from bottom tier.
Last place is Johto even though it's my favorite, weak Pokemon, no champ of their own, all the gym leaders' ace Pokemon are pretty middling at best, and the region feels chill as fuck with not many hotshot battlers, you never even fight your gender counterpart in HGSS.
Also I'm not including legendaries because that's a whole other discussion and lmao Arceus.
