Alright, I'm gonna go with BW2 I think. Now the hard part.
BLACK OR WHITE? BLACK OR WHITE? BLACK OR WHITE?
I have no idea which one has what pokemon.
Edit: Looking at a list, I don't know most of these exclusive pokes because I skipped gen 3 and 4. Any objective "Black/White2 has cooler pokes imo" opinions are welcome.
I didn't care about any of the version exclusives too much... all the Pokémon I liked were in both versions, and there were even some in-game trades that allowed you to get some of the version exclusives, so it wasn't a big deal.
Also I think you should play Platinum even though you've already decided on BW2 D:
Even though BW2 is very good too.
I personally played White 2, btw. But that's because I played White 1 as my "main" first game.
If, after I play B/W2, I still want to play another Pokemon game, I'll play Plat just for you.