I don't have a favourite but I do have one who can be more difficult than the others, and I can see how at times she has been the least favourite.
I love all my kids equally but to be honest there are times I don't like them equally. But it passes.
I've known heaps of people who grew up in families where there was a known favourite, perceived or in reality. I know my brothers, because of a variety of mental health problems required more help from my parents than either my sister or I have needed and it seemed at times like they were the favourites, but now I am a lot older I understand my parents were only trying to help those who needed it most at the time.