I have a favourite child. The family dog. He's obedient, loves me unconditionally, doesn't stay out all night, doesn't answer back (although he grumbles a fair bit now that he's getting on in years) and has grown up alongside both of my actual children...
When my son was younger he asked me outright whether he or my daughter were my favourite child. I said "neither of you - [the dog]'s my favourite child, but ssh... don't tell him!" So it's become a "thing" now. I love both of my children equally. As other posters have said, they both have different traits which endear them to me, as well as similar ones that also make me proud of them. They're individuals in their own rights, as well as my children, and I respect them for these things just as much as I love and, actually, am irritated by their quirks.
My mother has a favourite child - DB2, who is 10 years older than me (I'm the youngest of 3, with DB1 being 14 years older than I am). Whilst DB1 and I both went on to university and careers (he's a paeds consultant, I'm a forensic archaeologist), and formed long-term relationships, DB2 drifted, didn't achieve a particularly great education (he could never decide precisely what he wanted to do), has no friends, is on his third marriage (his current wife doesn't know about his first - and I suspect, his second wife didn't either), and has very recently scammed our maternal grandmother out of her house and the money our grandfather left to her. His son literally moved to the other side of the world to get away from him, because - according to my nephew - "he's a narc through and fucking through". He also doesn't know about the fact that he has two granddaughters, not the one whom my nephew's (now ex) girlfriend belatedly informed him about. My ex is also his mother's pandered to favourite, the one who didn't achieve anything academically, has an inflated opinion of himself, who can't cope with being told "no" and who has tried to introduce the "favourite child" system of exclusion to his "old" family and his "new"... and can't understand why our son has a very low opinion of him for doing so!
Being the favourite child doesn't do you any favours in life. If anything, in my opinion, it actually hinders you, and stunts your emotional growth to the extent that you can't function properly as a responsible adult.
The family dog, on the other hand, simply revels in anyone paying attention to him and loves all of us irregardless.