>>27493702Johto's arguably the best; the only thing that drags it down is the bad level curve. Even HGSS didn't do enough to be able to fix that.
I think the main reason it feels better in Johto is because they just have Kanto and Johto Pokemon, which is still retreading old mons, don't get me wrong, but it's less noticeable. On top of that, most of Johto has a lot of fun visual designs that usually have some purpose to them; Violet, Ecruteak and Cianwood have the older Japanese architecture, Mahogany and Blackthorn are very rustic (and in Blackthorn's case mountainous), and Olivine is the port town. The routes are usually pretty straightforward, but offer alternate options that let the player either take the fast path or to explore a little (Mt. Mortar is one of the best examples, as is Hoenn's desert).
That's kind of what Kalos and to an extent Sinnoh lacked in spades, actually. It felt like you had to tread so much ground in both of those games that my interest mostly plummeted, and a lot of the optional areas that there were weren't set up in a way that helped make them be for backtracking and exploring at once. That weird underground ruin in the autumn-themed route is a good example of that, it's actually pretty memorable but you can't really take it from one end of the route to the other as an option, you'd have to stop in your travels, look around, then resume where you left off. Autumn route not being too confusing is what saves it.
