the apple of (one's) eye
A cherished or favored person. This phrase is thought to be Biblical in origin. My nephew is just the apple of my eye. He has five kids, but his only daughter is clearly the apple of his eye.
apple of (someone's eye)
Fig. someone's favorite person or thing; a boyfriend or a girlfriend. Tom is the apple of Mary's eye. She thinks he's the greatest. John's new car is the apple of his eye.