This is a tough question. It's like asking who is the best character from The Great Gatsby. The simple answer to that is "none of them." They are all corrupt in some way. The same could be said for The Guide. The men in the story are cold, abusive, dishonest, etc. The women aren't much better. Raju's mother might be a good choice, but she is a weak character. She is not central to the story, and she is incapable of making decisions on her own. She needs a man to do that for her.
I'm going to go with Rosie. She's no angel, that's for sure. She cheats on her husband with Raju, and is forced to live with him when her husband leaves her. She's also quick to walk away from Raju as their relationship cools off. So commitment isn't a strong characteristic of Rosie's. However, she is an empowered female. While she is a traditional temple dancer, she is also extremely well educated. She has a masters degree in economics.
Another reason I like her for the "best character" role is because she has dreams, and she actively pursues them. She loves dancing, and is willing to do what it takes to succeed there. At one point in the story, Raju demands that she avoid being around other artists. Rosie flat out resists.
Lastly, I think she is a great character because she is fairly wealthy by the end of the story, but attaining that wealth is a bonus. It's not the main goal. Her dream and happiness is the main goal. Making money at it is extra.