I was having this discussion with a friend. Most parents would claim to treat their children equally and not have favourites. However, I think most parents have a favourite child, it may be different for mum and dad. In daily life you have to make decisions over which child to attend to first, you may subconsciously nurture or cuddle one more than another etc.
Most of my friends have said that either they or one of their siblings was/is preferred by a parent, in my own experience I would say it was clear who my mum's favourite was and who my dad's was, but maybe this is clouding my judgement 
Aibu to think parents have favourites and though they shouldn't show it, you can usually tell who it is?