My favorite is the one who isn't crabbing at me! Some days, I have NO favorite!
Seriously, there are so many different facets to a child, you can appreciate them in so many different ways. For example, DD10 is a thin, blonde dancer--we're still scratching our heads, I have a distinct recollection of giving birth to her, but let me assure you, I'm not thin or blonde, and don't dance! OTOH, she's also very smart, takes karate, loves forensics, and the bus bully is afraid of her, so she's probably my kid after all!
Her two siblings have their own good and not so good points--DS8 is very sensitive, which can be good or can be a PITA--it's just the way he is! And DD2 is adorable, but, well, 2!
We have another boy on the way. The only one bummed about that is DS, who thinks he won't be so special anymore. But he's my special "Buzz", nobody else can be my special "Buzz".
I know my mom and my MIL both have favorites among their kids. My mother doesn't favor any grandkids (of course, they're all mine, less of an issue). MIL does, but she favors whichever one is the golden child of the moment--for years it was one niece, then another, now it's DD because she's in the gifted program. Which is nice, but we loved her when she was bald and toothless--being smart is just icing on the cake. It gets really annoying even when it's your kid being favored (are my nieces now chopped liver?).