Whenever I have a new friends play the Pokemon franchise, I always make them play a game from every generation for them to compare how the games actually..."EVOLVED" . It's also better for newbies to get acquaintance to Pokemons of each generations.
So usually, I have them play FireRed/Emerald/Platinum/Black
I don't want to repeat what most already said but...yeah, if you don't care a bit about what's happening in the game; plot, flashbacks, and so on...then it's OK to start with B2/W2, though it's more fun if you play the prequels as I am certain you will ask for more Pokemon once you finish.^^