A parent simply loves his/her kids in different ways, and for different reasons. And thats too depends on parent's choices, circumstances etc. And sometimes it just happens naturally, that you give much affections and attention to one of your kids than another ( some kids are Easy to get along with)
Its just like u have your favorite kid on certain days. Eg. U real like a certain type of food, and whoever serves u that favorite dish of yours on that particular day, will be your favorite kid on that day etc
As a parent, u ain't supposed to show that Ur kid XXx is ya favorite kid. Your kids shouldn't feel like they have to compete for your affection and attention.