That's an extremely difficult question. It should be easy, because you only need to pick one and you've got 4 choices, but each is tough for Clinton in his own way and each presents some opportunities for attack. I feel I need to start at the opposite end: Who would she least like to run against? Even that way, it's difficult.
The link goes to my son John's Facebook page, where some people (not John) are saying Hillary would prefer to go against Trump. I don't see how that can possibly be right. I think Hillary is terrified of having Trump attacking her in his strange, unpredictable way. How can she prepare? How can she respond? These won't be polite attacks to be fended off with her trademark chuckles and guffaws and claims that she was already previously vetted. It will be wild, and tremendous energy, strength, and adaptability will be needed for the fight. And part of the attack will be that she lacks the energy, strength, and adaptability to be President, so her difficulty fighting him will fuel more attacks.
I'm going to guess Hillary would prefer that nice Mr. Kasich.