Seeing as this is in Gaming Central I'm assuming the best functionally rather than personally? Because if so, oh god I'm glad Unova isn't winning because that's easily the worst and B/W's biggest problem. Insultingly small and ridiculously linear with incredibly boring and bland environments? Not to mention it goes against exploration and is incredibly formulaic? Yeah, easily the worst designed region in a Pokemon game. Also, it's a mess, and makes absolutely no sense. That desert area is there because it is. Not once in Unova have I ever felt "oh, this place is great, maybe I'll stay here and explore before moving on", and it's not only because they're not as engaging and intractable, but it doesn't ALLOW you to due to how linear it is. Also, half the region is filler for post-game.
I'm glad Sinnoh is winning the poll. A huge, immersible region with varied environments and tons of hidden details and secrets? It's also such a joy to explore and dive into. Shame it's stuck in games that are horribly paced. It's the anti-Unova. Second place would either be Hoenn or Kanto because both allow complete freedom and satisfyingly rewards you for exploring. It's kinda hard to choose which.