Well for the men I just don't see anyone other than the two I mentioned previously winning it. Roger hasn't played since IW
and clay isn't his best surface anyways, while I saw Murray play in Monte Carlo and he wasn't very convincing. Novak definitely is a favorite because Nadal doesn't seem to do really well in Madrid and he might use the confidence from the Monte Carlo final to win this title (but I think it'll be a Rafa-Novak final).
When it comes to the women I'm more unsure. Theoretically it should be a Serena-Maria final but Serena hasn't made a clay final since 2002 so obviously it's not that simple. However I think that if Serena doesn't make it to the final then it'll be Li instead. I can't see Vika in the final (I think she'll make it to the SF) clay isn't her best surface and she hasn't played in a while so she probably will be rusty. But if Serena doesn't make the final I'll be disappointed if Maria doesn't win this.