It's not enough just to say that a map should be creative, or natural.
A good map has to be playable, it has to stay true to the Pokemon style, and most importantly,
it should feel like a Pokemon game.
The maps in a game, make the overall feel of a region, your towns should be like towns, they should be vibrant, each one with a flare all it's own, with ought going over the top.
The Hoenn region (which to me was the most well mapped region in the pokemon franchise) was nothing short of incredible. I wanted to explore it, I wanted to re-visit places I've seen before, I wanted to find hidden areas, hidden items and, hidden tracks for my acro bike (HELL YEAH) which truly made it a memorable experience.
And that's it, the map, should be an experience all its own.
Corny as hell I know.