Everyone is different. However, in my experience as a parent I cannot imagine playing favorites. This will become a problem for BOTH the child who is NOT the favorite and for the child who is favored. Why? B/C the child who is favored will experience feelings of guilt; the child who is NOT will have feelings of resentment.
In the end, your children will not build close relationships with each other. If there are two -they will always feel the separation of favoritism. If there are more--the children who are not favored may 'gang' together and ignore the favored child.
While this may not be a big problem while 'mommy' is around to manage the behavior, think of a time when you are no longer living and your children rely on each other for support and love. The 'favored' child is now odd man out b/c trust me...his / her siblings do not see this child in the eyes of the mother.
I have had two women (one being my mother in law) tell me that I MUST prefer one or the other of my daughters. I could not comprehend this. It is beyond me. My response? I love my children each for the unique people that they are. Do I favor one? No. Do I love them differently-yes. Why? Because they are individuals. Love is unconditional. Period. I accept and love my children and grandchildren in the way that they are: special and unique.
Does favoritism make you a 'horrible' parent, no. But, I hope you look into your heart and ask yourself what it is about this child that you favor. You may be surprised by the answers you discover. Then, ask yourself if there is something that you can uniquely enjoy about your other child/children. God Bless.