I was once told by someone who is a parent that every parent has a favorite child, and that you should always try to hide that fact from everyone. I am not a parent, so I can't say I have been in this situation. But my gut tells me this is the wrong way to go about it.
I think that the way you treat your children should be the same way you treat anyone else you are close with - as an individual. Sure you get along with some people better than others, but that doesn't mean you can't love them all equally for their own unique personalities. It's like comparing apples to oranges. If one child is a great athlete and the other is a great writer, you don't turn to the athlete and say, " I sure wish you were as good a writer as your sibling." or vice versa. You go to the events of your athlete and you read everything from your writer. Even if you yourself are more interested in athletics than writing, it doesn't have to take away from your love for the child who likes to write. You can still get excited about each story he or she writes. You may have more in common with one child, but I don't believe that should take away from your love of the other children.
This is, of course, a simplified example. Personalities are very complex and varied. However, it still stands that you can love a child for many different reasons, or even for no reason other than they are your child. Love should be multiplied, not withheld. If you don't feel you have enough love for each of your children, then that is a personal problem. Hiding the fact that you have a favorite won't work because sooner or later, everyone can see through your lie. I would suggest that you work on yourself rather than trying to fool everyone else. Work on having unconditional love.
Maybe I would feel different if I had children of my own, but at the present I couldn't tell you that I love any of my family or friends more than the others. They are all very different and very special to me, and I could never care more for one than I do for them all.