I must admit I can't see how any writer can avoid having favourites amongst their characters. There are bound to be creations we prefer over others, simply for things such as we like Character A's sense of irony, which Character B, noble as they are, simply doesn't have. What DOES matter is that we are scrupulous about how we create our characters.
By this I mean when planning out characters, we should ensure each and every one of them has flaws and virtues. Each and every one of them must have good reasons for acting the way they are. Each and every one of them should feel real to a reader. No cardboard cut-outs here!
You, as the writer, have got to know what makes them all tick. You need to know what drives them, what would frustrate them, what would tempt them away from the path they're supposed to be on, and how they handle weakness in themselves, yet alone in others.
A good sign of a "proper" favourite character is knowing you've created a character that for many reasons you dislike (e.g. you disagree with their attitudes) but have brought them to life in such a way your reader will be intrigued by them and there will be no sign of your antipathy towards them either. Good luck!
Image Credit: As ever, the images are from the marvellous Pixabay. Captions over on the CFT post.