I love both of my children the same, but get along far better with my youngest (son) than I do with my oldest (daughter). However, I've always said that my favourite child? Is the dog. Every time they asked me which of them I loved "the most" or "better"... my go-to answer was "the dog".
Our dog at the time was easily loveable, though. A complete wuss, adored both children to bits, was genuinely pleased to see me, even if I'd only been asleep and awoken (often to find him creepily looming over me like toddlers tend to...).
My answer still, even though my children are 16 and 25 is "the dog", because my current hound is batshit crazy, but laughs at something at least once a day, chases my 16 year old son around when he's being lazy (this morning she was "shouting" - or barking very crossly - at him because he was late getting up, it reminded me almost of a mother yelling "socks! Where are your trousers?! Why are they on the stuffed hippo and not where I left them?!") and just generally reminds me that life's still worth living.
So, it began as an effort to stop my children from fighting, but seems to have become a genuine thing. My favourite child is the dog (who is treated as a dog, not as a surrogate child, I should probably add!).