Mechanics-wise V > IV > III > II > I. Apart from contests and Hoenn's BF features, I don't think we're missing anything in B2W2. Everything else has been improved greatly.
Region-wise... I do like Sinnoh a lot. I don't like Hoenn though. Unova is pretty good. Johto and Kanto are in awkward situations where the latter is just dated and the former was never fully realised. Johto's lore and legendaries are great, but the region is the smallest to date.
So maybe Sinnoh > Unova > Johto > Kanto > Hoenn. I'm currently 32 episodes into the DP anime and plan to watch all of it and the four movies, and so far it's been the best series out of the 4 so far, which is making me like Sinnoh more than usual.