I have two brothers and growing up we did used to joke about who was the favourite but I genuinely don't believe that my parents had one and I think my brothers feel the same.
My dad is closer with the middle child and my mum is perhaps very slightly closer to the youngest and oldest but I definitely don't believe they love or even like us different amounts.
My DH is so blatantly the favourite that it's uncomfortable. Both he and his sister are very aware and when it's mentioned his parents haven't ever even really argued with it. Not only is he very obviously liked more but he's treated differently too. It's quite extreme.
My DH does not seem to think that this is weird at all. He acknowledges he is the favourite but has the attitude that "someone has to be". He thinks all parents have favourites and that anyone who says otherwise is lying.
I don't think he's right but I do sometimes doubt myself. I really don't think my parents have a favourite but they wouldn't tell me if they did.
We have one DS at the moment but both of us would like more. However if it really is this big secret truth that everyone has a favourite then I'd be a bit nervous of having another as it just seems so awful.