My grandmother pretty much resents my mother because she (my mother) took grandpa’s attention away from grandma. IOW, she saw her own daughter as a rival.
I think some parents have obvious favorites, but I think those type of people have emotional issues anyway, and the way they treat the children is just an aspect of that.
I was watching a comedian on MTV’s Half Hour Comedy Hour (which was about 10 years go, when I was in college) and I still remember this one guy who was talking about this very thing. He was one of 10 children, and when he asked his mother if she had any favorites, she replied “Well, I have 10 fingers, and I wouldn’t want to lose any of them.”
His response: “Yeah, yeah…but who are the THUMBS???” 
That still kills me to this day.
Jesus. Your mom said that to you (or your sibling), hillbilly queen? I think I’d start bawling if my mom said that to me as an 8 year old…or even now. I mean, it’s an explanation, yeah, but I would prefer to believe myself perfect in every way.
A lot of you are saying you think the oldest child is the favorite in your family…Hm. In that case, I think that’s me. 
Though I don’t think I could do it that casually…I don’t think I could say something like, “yeah my brother is the favorite child.” I’d just feel so emotionally outraged.