I am able to admit that I have really $hitty knowledge - as of now - about bike races because I more or less don't watch them since Armstrong retired and some serious $hit hit the fan about his and other participants dope habits... Funny thing though - all the other things I frequently watch probably full of dopping too (as all professional sports), but somehow I've never stopped following them.
For me Nibali is the hot favorite there but I would not ignore Valverde as he seems to find a good form during the winter classics... he is the one to watch...