That will be highly individual as people hold their fat in different places. Thus, someone can look good with 15% bodyfat, while another individual may need to be down to 8 or 9% to really feel that they look good. Muscle mass is important as well, the more lean mass you have, you can "get away" with a slightly higher amount of fat.
In terms of health, I don't know, I have never researched what is optimal from a health and longevity standpoint.
James 1:16-17 ESV
Do not be deceived, my beloved brothers. Every good gift and every perfect gift is from above, coming down from the Father of lights
With God's help...Mens sana in corpore sano