Well I can't say why Juliet is your favorite character - only you can say that - but why don't list some of her positive character traits?
Juliet is a responsible young lady in many ways. She listens carefully to her parents and does her duty as they ask her to. She is concerned with practical things even when her emotions are high, telling Romeo he is putting himself in danger when he comes into the orchard. Even though she deceives her parents and marries Romeo, her attentiveness to her new husband shows her to be a responsible Elizabethan lady. She is doing her duty as she is supposed to by making her husband her top priority.
Juliet is also an insightful and intelligent young lady. At the party, she cleverly disguises her interest in Romeo by asking the Nurse the names of many of the young men. When Romeo arrives, she talks in depth about the behavior of men in love. She shows her understanding that emotion can cause people to make promises they will not keep, and she insists that Romeo be sincere and swear not by the moon, but by his own self.
Her relationship with Romeo is impetuous. She quickly falls for him, and while cautious at first, gets swept away in the heat of their attractioon. She has more control in the relationship between the two. She dictates how he should speak to her in the balcony scene (telling what to swear by) and she controls the details of their marriage.