I have played all of them, and for me BW2 and Crystal were the best I've played.
BW2 has beautiful graphics, an amazing storyline, and a world tornament (in which I appear :3).
Crystal, was the prototype for most of the amazing features included in the DS games, Moving sprites, wireless conection (though it was only included in the Japanese version), Male and Female characters, and event pokemons (I believe there was an event Mew in the first gen, I am, however, not entirely sure on this), and lastly I appear in this game as well :3
To sum things up, I had fun playing each game, but the between the ones I have mentioned above I have to choose Crystal because it offered me very pleasent moments during my childhood, BW2 comes as a close (very close) second place, followed by Platinum (I really enjoyed the storyline in this one almost as much as in BW2).
Thanks for reading my silly nonsense XD
Gym Leader Whitney~