I grew up in a wonderful home with both parents. I know they both loved me, but I was closest to my dad. We were best friends. He supported me more than my mom and showed so much interest in my friends, hobbies and interests. When he died I suffered tremendously, having to take leave from College for a year. I still miss him and to this day, over 15 years, I haven't gathered the courage to visit his grave.
I have children of my own and their dad and I love them both. I have to admit though that one of them makes me smile a little extra each day. Parents will not have favorites as a matter of choice, but children could have different personalities and charisma that parents might find more alluring than others. Its just the nature of things. We might love our children unconditionally, but no two children are equal. They all require personalized emotional care.