If it were not for Kanto, G/S/C would have been boring games. The Johto region does not offer enough to warrant it a standalone. The Rocket subplot was not fleshed out enough (not even in HGSS). Obviously RBY are the worst games on this list, but nostalgia will prevent a lot of votes.
I voted for DPP, because I did not like their changes to Hoenn's contest formula. :P That, and even six years later Sinnoh still feels like a labyrinth. But I do very much miss the Pokétch.