1. Hoenn 10/10
2. Sinnoh 9.5/10
3. Kalos 9/10
4. Kanto 8/10
5. Johto 7/10
6. Unova 4.5/10
Hoenn is awesome. My first game ever was Pokemon Emerald, and I've considered Hoenn to be my favorite region ever since. I'm excited for AS and OR!
Sinnoh is another very good region. It was very spread out which I liked, and Diamond is still one of my all time favorites.
Kalos was pretty great as well. They did very nicely on it, which was a nice surprise after Unova.
Kanto is good, not great though. Sure, it's nostalgic, and it's a pretty good region with a pretty good difficulty curve, but it's not on the level of the first three.
Johto is very meh. I enjoyed playing the Johto games, but the region itself is very meh. Don't get me wrong, the gameplay and storyline from Johto was great, but I would have preferred a two region game in some other regions rather than Johto and Kanto.
And finally...Unova. Oh my word, it's like the game designers quit. At least half the new Pokemon were dumb, and you couldn't catch any in the main story in B/W. Then they compressed 8 gyms into 8 cities! So stupid! By far, the worst region ever IMO.