I believe this study. I'm yet to have my 2nd study so currently dd is my favorite as I don't know the newbie yet.
My mother however. I now she loves the four of us equally. However she likes us different amounts. My sister and brother both claim I'm mums favourite. While its not true in the way they mean it. It is true. Mum and I get along the best. I don't hurt her or disappoint her like the other two do. I'm 30 and mum and I have a adult friendship relationship now while my brother (25) and sister (20) act like they are still in their teens. I was the easier child to raise and in some ways mum and I grew up together as she was 19 when she had me.
My dad clearly favours my younger sister than me over my brother. My sister over took me when mum and dad separated dad blamed me and didn't get over it for some time fractured our relationship and he moved the favorite to my sister. Where once were very close.