I really like Johto. It's not the most geographically complex region, but I feel like it has the best natural flow out of all of them. By that, I mean that things change gradually and feel a lot less arbitrary than in some of the other regions. Kanto, while also feeling geographically similar, feels a bit more sparse, which makes sense given that it was very early. The others - while I like Hoenn and Sinnoh a fair bit - feel slightly more slapdash, in my opinion.