Wow, this is a hard one. My favorite of the family that has been unusually cruel to Harry for 10 years of his life and even more considering how they treat him when he comes home during the summer. I know without a doubt Vernon is not my favorite. He's the cruelest to Harry, blaming him for every thing that goes wrong. There is no physical abuse, I don't recall, but there is a lot of mental and emotional abuse most from Vernon. How Harry came out of that situation without any real damage is beyond me.
The choice is between Petunia and Dudley. They're not good people. They're not nice people. They are mean to Harry. They are selfish brats that care about themselves more than they do Harry or in Petunia's case, her sister Lily.
Dudley does come around an extremely small bit in Deathly Hallows, though, so I guess I would label him my favorite? He's not really a favorite, but I guess he stands out more in his family. He is a big fat spoiled brat, but he did tell Harry that he wasn't a waste of space. He even left Harry tea outside his door, which Harry didn't drink because he thought it could have been poisoned. Harry saved Dudley's life from Dementors two years ago, but Harry didn't know Dudley's attitude had changed towards him because he hadn't spent any real time with him the summer before.