If you didn't play the games when they released, it's hard to understand their appeal. To anyone that didn't, it might feel like they're missing something, but for those that played it on release, every game since has added things that make the original experience better.
HeartGold and SoulSilver possess abilities and the physical/special split, as well as the running shoes and Pokemon following mechanic (which is really lovely, despite having little use), and in that way, it's the closest to "modern" Pokemon you can get. But Gold and Silver before it were the definitive trek through Kanto, to me, despite the region being smaller.
In terms of regions having "character", I would agree that Sinnoh has the most personality. Mt. Coronet might be a massive obstacle that splits the world in two (or three, really), but it is this feature that you constantly return to and gain new ways of traversing, and finally reaching its peak feels like a very great accomplishment, in terms of the franchise. While the water of Hoenn felt more like a nuisance because of its size (and the ridiculousness of Dive), Coronet was just a big feature. Not to mention, the different areas around Coronet had tons of variety and personality in themselves. It didn't feel like these massive, inexplicable terrain differences- the Northern region was predictably snowy, the Western region was tame and flowery, and the Eastern region was a mix of marshland and shores.
Man, I want to go back and play those games.