I would vote on 5th Gen personally.
Like with G/S/C there were 2 "major" legendaries(Lugia/Ho-Oh) and 3 sub-legendaries(Dog Pokémon). They made the 2 games around Ho-Oh and Lugia but they didn't have much of a story behind their purpose. Crystal introduced the "storyline behind legends" part of the game. Now, in D/P, they made the game revolve around the 2 "major" legendaries(Dialga and Palkia) and had many sub-legendaries such as Rotom, Darkrai, and Giratina. Back to Crystal, it was based on a continuous storyline to find Suicune(a roaming legendary). The only real option that Gamefreak would have to base another D/P game on would really be another triplet Pokémon. They created Triplets but in my opinion they aren't very good. Azelf, Uxie, and Mesprit, I don't think, would be very good candidates for a game based round them. Not enough of a storyline in D/P to go on. G/S gave enough info to base a game. Suicune is a fast running, roaming legendary. Moo...
If no one sees my point than HOORAY! I dont give a crap. kthxbai :)
~Gengazard