Hard to predict, it all depends on the draw.
Serena hasn't made the F of Roland Garros in ages so I'm not so sure what reaction, emotions she'll develop if ever ever she reaches there... this could turn out to be like a Safina Kuznetsova encounter in 2010.
Sharapova hasn't defended a single major in her career and the sole fact she bailed out against Errani in Rome where she was the double-defending champion is a bad indication about her chances in this slam.
Personally, I'll go with Stosur. Has already defeated Serena in Roland Garros and in majors overall, has already made the F in RG, her game is tailor made for the surface with all her spins and she seems to have found her level of play (forehand) again in Rome.
Upsets will be, Kvitova (2nd round), Li Na (2nd round), Radwanska (3rd or 4th round) and Wozniacki (1st round)... and hopefully my bb Victoria reaches the SF, that'd be a reasonable achievement for her. Errani as for her, I predict a 4th round or QF showing at max.