Black/White 2 are actual proper sequels to Black/White, unlike the traditional "third version"; BW2 also have a TON more content than pretty much any other Pokemon game, previous or future.
I'd highly recommend BW2; they're the only games to have a Challenge Mode, to have the Pokemon World Tournament (where you can fight older Gym Leaders and Champions), to have the Black Tower and White Treehollow. It also has seasons and hidden grottoes (for the occasional Hidden Ability - most of the time it's just items, but when it's a Pokemon, it will have its Hidden Ability), for extra flavour, along with PokeStar Studios for a fun little distraction.