On paper it's definately JJ, but going with current form I'd pick Lucie Safarova for Amelie Mauresmo, it's the most likely surprise.
I really hope Olaru can prove to be a tricky opponent for Ana as well, but she's still very young, so she has pleanty of time to get better even if she loses tomorrow.
Also, Safarova could be dangerous to Mauresmo. Santangelo, Stosur and Krajicek are worthy opponents, but I'd be shocked if they gave their respective TOP8 seeds a very tough time, let alone cause an upset. I never even heard about the other three. Ok, Cibulkova and I guess I'd seen that Romanian name before in the juniors draw, but never saw them.
remember you also have to take into account the actual player and how their game may not be suited to their opponent or if they have a mental disadvantage against their particular opponent...this is not just a poll about who is the best out of the 8 players!
Venus may be the least tough opponent specifically for JJ if she had a 0-20 h2h against her and her game was one which Jankovic loved to play against and Jankovic mentally had an edge over Venus
Kudryavtseva may be the most tough opponent specifically for Masha if she had a 20-0 h2h against her and mentally she believed Sharapova could not beat her and Sharapova couldn't cope with the type of game Kudryavtseva had!
Jankovic v. Willaims
Mauresmo v. Safarova
Henin v. Santangelo
Vaidisova v. Stosur
Williams v. Krajicek
Ivanovic v. Olaru
Kuznetsova v. Cibulkova (I would love her to win, but it isn't going to happen)
Sharapova v. Kudryavtseva