What would you think about me saying no to dating my best friend?
Is there a reason you don't want to date them?
Because I am not into her like that and I would rather remain friends.
That's good that you want to be honest about this. At least you aren't leading them on so they should respect you for telling them the truth.
I'm planning on making it clear that I'm not interested and end things in a good way.
Yes, it's good to be honest with your friend. Hopefully they will respect that.