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As far as region design goes Hoenn is definitely my favourite. I love how the region opens up organically as you play through it, with paths you've seen before but couldn't access slowly opening up. Stuff like making you go to Dewford/Slateport by boat and showing you a route you can later surf through. Being able to surf from Slateport to Oldale or Pacifidlog and thus the ocean. Rustboro being connected to Vedanturf and Fallabor through tunnels that were previously inaccessible. The desert opening a quick route that had to be done through fiery path before.
And as OP said, every parth of the region has such strong identity too and the region does a good job at feeling very large. Making you go through fiery path and then the sooth-covered route before climbing Mt. Chimney itself was fantastic. The path to and from Fortree City is extremely aesthetic with the rain through the jungle, it makes you feel like you're travelling through the middle of nowhere. The ocean feels massive to the point that I could get lost trying to find places like Soothopolis as a kid, but it has great places to explore like Pacifidlog or just little islands and caves to check out.
It really just comes together and feels whole as a region and rewards exploration and going back to check out new routes. No other region captures the feeling of adventure quite as well as Hoenn to me.