I'm not sure how this will be received, but my personal favorite region is Sinnoh. I love the layout and story progression of the Sinnoh region (Platinum > DP though, 'cause that gap between Gardenia and Maylene is so unnecessarily long. Fantina being Gym 3, and then having the choice to go to Maylene or Crasher Wake for Gym 4 is much more interesting/fun).
Even though Team Galactic is a bit of a mess, Cyrus alone makes up for it with his insane crackhead energy, which I love. All the Gym Leaders are fun characters (they actually have some depth to them), and Cynthia is arguably one of the best Champions we've ever gotten. I personally like all three starters, and I like quite a few of the new Gen 4 Pokemon, although I'm not a fan of all of them (some are pretty dumb/useless).
The one downside to this region, in my opinion, is the post-game...There really isn't anything to do in Sinnoh once you beat the league, other than the Battle Frontier and completing the Pokedex. However, this region has all the nostalgia points because Diamond was the first Pokemon game I ever fully completed (Leaf Green was my first Pokemon game, but I never beat the league until YEARS later when BW2 were about to be released. Diamond was the 2nd game I ever had).
