My brother was favoured over me (though parents deny it, they do so in a 'we-know-its-obviuos-but-wont-admit-it' kind of way!)
Anyway, it never really bothered me that much, it was just the way it was. my parents were extremely kind and loving to me, its just that i was the rebellious one, didnt follow the path they would have wanted, and my brother did. I wasnt as clever as him or as well behaved, and he was involved in lots of things my parents could enjoy and get involved in, like sports.. whereas I just liked to go out with friends!
Anyway - my brother was very aware and didnt want me to be left out (he's younger by the way) so as a result we are very close. Always have been. He would stick up for me all the time and make sure I didnt feel left out.
So its not always a bad thing to have a 'favourite'. It ended up okay for me as it made me and my brother so close.
Do you watch Ugly Betty? 2 weeks ago in the episode was a thing about how bettys dad 'prefers' her to her sister. Its a good example of how it can be okay I think.
As long as you are brought up in a loving environment and not blatantly excluded, then its okay to have one that you slightly 'prefer'.
(Sorry its very hard to word this without it sounding a bit strange!!)