It's all subjective. Usually, right underneath the head. The area called the frenulum. But other than that, it all depends on who you're talking to.

I have found that the area under the head and around the bottom of the head are very sensitive. Also the underside of the shaft is sensitive. These are usually the key areas to apply the tongue and to cover is saliva when sucking a penis.
Wendy

-the area between testicles & sphincter.
-underneath the head
-the edge of the head
-the foreskin just below the head