It has to be Minerva for me. I love the guts that she shows when she slaps Trujillo and then suggests the gambling session for her right to study law. She shows most pluck out of all the sisters and remains determined to resist Trujillo throughout the entire novel. I also like the way that she organised for her half-brothers and sisters to get a good education.
My favorite sister is Mate because as she grows up through the book you see such a distinct maturation of her character. She starts out with childish diary entries and writing about clothes and boys. Even her initial joining of the rebellion is to impress the guy, but as she works with her sisters, she comes to see the necessity of what they are doing to bring about change in the Dominican Republic. She is so strong, especially considering everything she goes through in the prison, and she is not broken by the abuse and loss of her baby. While Minerva is the clear hero, she is not a dynamic as her baby sister.
Mate because she just so sweet and has a little rebel inside of her (in the book). In the movie it is Minerva because she is so beutifull ;)