Which is better eyesight 20/20 or 15/20?

20/20. To make it easier to understand, 20/20 means you see at 20 feet what the normal eye sees at 20 feet. 15/20 means you have to be 15 feet to see what the normal eye can see at 20 feet. Now, 20/15 is better than normal. You can see something at 20 feet that the normal eye can see only when they are 15 feet away.
20/15 is better. 20 is the numerator. The smaller the demoninator number the better the vision. So 20/15 is better than 20/20.