Personally, I don't believe in having favorites. I think it isn't fair nor does it put the children in equal positions to each other. However, I do have children who come to me for additional attention, or ask more, or talk to me more. I do not go out of my way to contact them or give them more of my own volition. I can only offer, and those who want to, they come and ask.
I don't think it's okay, because it's only somewhat beneficial to the favorite - the other students gain nothing.