Basically, is there a Pokemon in your current team that you favor, if only just a little bit, over the others? It can be simply because of its appearance, strength, or a combination of different things.
In my case, I think my favorite Pokemon to use in my team is my Reuniclus. It has only been on my team for about a day now, but it's already proven to be pretty useful. I like the way it looks as well. Before I caught it, I think I would've chosen my Chandelure.