I should imagine Venus. Her powerful ground-strokes combined with her often excellent execution at the net mean she will be very difficult to beat.
However, her form has been somewhat lackluster of late, with her winning just 14 of her 20 matches thus far this season. Also, it has been known for her to lose early on at the French Open, once falling in the first round (in 2003, ranked world number 2, having been the finalist the previous year).
You can never really tell with Venus or Serena these days; they are not the omnipotent force they once were, but their triumphs over adversity in recent times show they are not only capable of winning tough matches, but they still have it in themselves to win at grand slam level.
I personally belive that, if Venus believes, (which she always does), then we should believe. Go Venus!!