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Diamond actually got me back into Pokemon, I felt disappointed after RS and quit temporarily, but I saw my brother playing Pearl so I decided to give Diamond a shot. The Underground is really fun if you have multiple people to play with, between setting traps to send them flying off and capturing flags to upgrade your base.
But I really did like Sinnoh's focus on mythology and creation myths, it felt like legendary Pokemon's feats weren't left in the abstract since you had library books and random people sprinkled throughout the region telling you their impression of the stories they'd heard about the Pokemon that shaped their world.