Sinnoh has awful design. Setting the repeated, boring, storyline elements aside, there is only 2 or 3 uses of Surf. Sinnoh is at least 90% land, with a gargantuan mountain right smack in the middle that shouldn't nearly be as big as it actually is when you compare the short amount of time you spend in there.
I liked Hoenn's water aspect. Most of the third generation was water, while Sinnoh is all about land. I think water evens out and honestly, makes environments look better. Hoenn had deserts, oasis', volcano's with ash, a city of ice, and tall mountains and tons of awesome terrain. Sinnoh's was all road and gravel. How boring.
Johto has many gems as well, in terms of terrain, but everyone knows that Johto had the best storyline out of every game. Plus, Kanto as well. Epic win.