1. Know that there are characteristics you will like better. Often times parents confuse the fact that they appreciate the ease of raising one child over the difficulty of another with preferring that child. It is important as a parent to distinguish to yourself the various traits and characteristics of a child, and that it is okay to like some traits more than others. It is normal for you to like some people more than others, which is why it makes sense that a child who is more pleasantly dispositional may be more likable than another child. However, you have to learn to separate the characteristics from the people themselves. For example, your oldest may be very hard to deal with because they are stubborn, and your youngest easy to please, which may make them more likable.
2. Learn to never compare children. It is important as a parent to never compare children, not in your mind, and especially not aloud. As soon as a child feels they are being compared to another child, their esteem will be threatened, sibling rivalries will arise, and more. Every person is different, and every child is different, so comparing one to another is totally unfair. Just is it would be unfair for someone to compare their parenting to your own. They are not raising your children, and you are not raising theirs, so there is no basis for comparison. If you remember not to compare your children, it will be easier to not play favorites because you will learn to better appreciate your children for their own specific characteristics and uniqueness.
3. Remember the phases kids go through. It is a simple fact that some people prefer the toddler stage to babies, or the baby stage to teens. It can seem like you are playing favorites just because you prefer the age that the child is at, not the child themselves. If you can keep that in mind, and not make it too obvious or difficult for the other children, you can help assuage some of your guilt. A five year old, for example, may be going through a very oppositional stage as they figure out who they are, and start socializing outside of their family, which may make it harder to deal with them over a one year old who is sweet and fun.

Playing favorites does not do anyone any favors, but do not beat yourself up if you find one of your children more pleasant to be around than the others. The fact is, there will be times and phases in all of your children's lives where you prefer them to the others. Just do not make a habit of making it obvious to anyone else.