Despite the USO win, I think the French is actually her best Slam, and possibly clay her best surface.
She's been far more consistent at the FO than other Slams in terms of "deep runs" - second week, 4th Round or better results. Of course on actual Slam title on hard courts is huge, and she does have more hard court titles overall among her 8 titles, but she's not one of those players who plays on clay outside of the clay season, even when clay tournaments are available.
AO - SF, 4R(x2)
FO - F, QF(x2), 4R(x6)
Wimb - QF
USO - W, QF, 4R
Maybe I'm going overboard, but she does seem to often get it together, at least to a large degree, for the French.