Which regions league is the toughest?

Question

If you have any thoughts on which region has the strongest league (out of Kanto, Johto, Hoenn, Sinnoh, Unova, Kalos, Alola (even though it’s not a formal league) and Galar (once it has come out)) then leave them down below.

Include the elite four and champion and no personal preferences please. Base them on movesets, levels, base stats and number of type weaknesses.

Comments

BW definitely has the highest level champion (excluding rematch teams). I unfortunately can't find speedrunners who defeated Alder, so it still might be easier to speedrun than USM.

---

It’s been a while since I’ve played through the story of BW so I could be wrong, but I’m pretty sure you don’t face Alder the first time you challenge the E4 in BW; after beating the E4, the Pokémon League gets taken over by Team Plasma and N serves as the final boss instead. It’s only when you challenge the E4 later that you face Alder, at which point the E4 has their rematch teams. So shouldn’t Alder count as a rematch team?

---

The Alder battle is the first time you defeat the entire League, and the question said "include champion". Also the highest level champion other than Alder is XY Diantha.

Answer 1

Highest level champion during the first battle: BW (Alder)
source
Highest level champion before the credits: XY (Diantha)
source
Hardest-to-speedrun champion: USM (Hau)
source (BW might be harder, but I couldn't find speedrun times for Alder in BW)