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I really enjoyed Hoenn because of the really varied activities and features of travelling, you could surf in the area around dewford town or take a ship there ride the mach bike up cliffs and jump down ledges on jagged path, collect ash to exchange for flutes in route 113. Also the sky pillar has the feature where you drop down if you step on certain tiles like in parts of Johto. It's probably the only pokemon game i enjoyed surfing in because they included so much content related to it, you could travel across rapids, dive, visit the abandoned ship shoal cave marine cave pacifidlog town sealed chamber, visit secret bases etc. with some of the other games i feel like surfing is mainly travelling to A to B while getting past swimmers (although there is a lot of that in Hoenn) the tidal aspect of shoal cave was a cool addition i thought as-well. Finally i liked the berry planting, contests, and going through the trick house.
Visually, Kanto I found slightly creepy since it was pretty monochrome, Johto and Hoenn had charming simplicity and Kalos has best visuals, sorry for long post.