I can't decide. I've spent so much time playing all of the games, I have no idea. :\ Sinnoh is probably at the top of the list, though. Johto's cool, but I'm not sure what made it unique*, so it's probably at the bottom.. But then again, Gold was the first handheld Pokemon game I owned, so maybe it deserves to be higher or something. I had always watched my brother play Red. It's definitely a tie between Hoenn and Kanto. I still like them all.
*Oh wait. There's the Burned Tower, the Celebi shrine/forest, Ho-oh/Lugia.... I still don't know. :D