Which third/upper version Pokémon game is the best when it comes to improvements?

I voted Emerald because the Hoenn battle frontier is my favorite battle facility.

But Platinum you could easily argue elevated Sinnoh to a greater extent due to the amount of issues in need of fixing present in the base game. And the addition of the Distortion World was very nice.

Crystal I think doesn't do as much. There's some missed opportunities with level scaling in Kanto.