I've thought about it and there's one nephew of mine that I'm going to tell is not my favorite.
Is there a reason you want to tell him that?
I have many reasons. I think he's a little priss and he needs to learn a few things about the world.
You shouldn't outright tell that a particular nephew is not your favorite as it may hurt his feelings. Have you tried to maybe spend more time with him? Perhaps in doing so he will grow up a bit and your relationship with him will improve.
I'm not concerned with sharing this with him and his feelings being hurt. I don't like him and I don't have time for him.
It's unfortunate you feel so harshly towards you nephew and I'm concerned about how you favor others over him. Telling him he's not your favorite will hurt him and you should take a step back and be more mindful and compassionate towards his feelings and emotions.