After re-reading my previous post in this thread, I realized that I was way too negative towards most of the regions. I mostly posted things I didn't like about them, so I'll try to write some things I like about them as well.
I have already said most things I liked about Sinnoh, but I'll say it again. Variation when it comes to environments. There are so many different types of areas. Regular routes, water routes, a flower field, swampy marsh areas, snowy areas, lakes and even a volcano. I believe it is only topped by Unova when it comes to environmental variation. I also really like most of the cities, most of them felt very unique and different from each other which I think is really great. The post-game was also great with many large areas to explore.
As for Hoenn, I really like the overall tropical theme. Made it feel rather coherent. It had several interesting areas like the Abandoned Ship, Meteor Falls and Mt. Pyre. It also had places with really cool designs like Sootopolis City. Even if it was mainly tropical, it managed to have some pretty good variation in terms of environments with things like a volcano which actually affects the areas nearby, which was neat.
Kanto had some cool areas like Mt. Moon (especially in the remakes) and the Safari Zone. I also like Cinnabar Island and the Pokemon Mansion, and I always thought Route 25 was cool because it is the last Route in numerical order and it has a dead end. Then I love almost everything about the Sevii Islands so they make the region as a whole better if they are counted as a part of it.
From Johto, I like the Ruins of Alph, Bell Tower and Lake of Rage. I thought those areas were rather cool and had good design points in some way. I liked Lake of Rage the best in the originals while I thougth the Bell Tower and the Bellchime Trail were better in the remakes. Regarding the cities, I like Goldenrod the best in the originals and Ecruteak the best in the remakes.
