I feel like Serena is the one to go with here. Vika had virtually no clay tune ups beforehand, and I don't think she is ready to take the French Open quite yet. Everyone else has been wildly up and down aside from Serena. While it is true, it took Serena this long to win a title this year, she did make the final of a slam and Indian Wells before winning in Rome. I think Kerber implodes with the pressure of following up her slam win, Aga well doesn't like the surface and that has to change in order for her to really do well here. Simona, well who knows who will show up.
I think the key here is where will the land mines like Daria, Keys, Stephens, potentially Kuzzy, potentially Kvitova fall. These are dangerous players if they bring their A game. If these players show up, they can cause a lot of issues for Serena, Halep, Kerber and even Vika if she is playing.