If I was purchasing based upon our current needs (but with the fantasy twist, of course), it would be Douglas Maicon - simply because RB is our worst position, and I haven't seen another RB who I rate higher than Maicon ATM. Too bad he plays for our nemesis...
Otherwise, if it's just for an out and out fantasy purchase, it's Messi... all day, everyday. He's definitely taking the Ballon D'Or this season... and he's still 21.