I have to agree that for me it's also a close call between the Mizukage and Hinata, although in the end Hinata wins for me. Although before the time-skip Hinata had this rather shy and bashful personality that fit her appearance, I felt that over time, especially after the time-skip it was akin to watching a flower blossom. Not only did she turn into a real beauty, but she also grown more confident, and considering the challenges she faced (After all, it was she who went up to challenge Pain in an attempt to save Naruto after he went down.) and the trials she had to overcome, the fact that in the end she even succeeded in getting noticed by Naruto was a well-deserved reward!
All in all I find that Hinata is the prettiest, not just in appearance but also the overal personality and the development as a character as a whole.