I am studying a non-medical field so please bear with me.
Which part of the flat skin has the most nerve receptors?
I plan to build a grid of vibrators.
I did not find a reference in English, only a German:
Die Fingerspitzen weisen die höchste Rezeptorendichte auf. Damit wird Begreifen möglich.
Lang, Phillip, Lang, Florian: Basiswissen Physiology, Springer-Verlag, 2007; Seite 376
The fingertips have the highest density of receptors. This is why grasping is possible.
Translation into English
The all over highest density in receptors can be found in the Fovea centralis.
The largest organ in your body does -- the skin.
The most sensitive area would be any distal (meaning the farthest away) extremities. Areas that require fine control (such as dexterity). Your finger tips or genitals would be #1.
An erogenous zone (from Greek ἔρως, érōs "love" and English -genous "producing" from Greek -γενής, -genḗs "born") is an area of the human body that has heightened sensitivity, the stimulation of which may generate a sexual response, such as relaxation, the production of sexual fantasies, sexual arousal and orgasm.
There are two types of erogenous zones: nonspecific and specific. Those of the nonspecific type depend upon exaggeration of a basic tickle sensation. Specific erogenous zones, the mucocutaneous zones of human and animal, have special neural and cutaneous anatomic characteristics. The mucocutaneous end-organs in any given species appear to be identical in all of the the zones. The endings of the primates are markedly different from those of lower animals. Development of the nerve endings is principally postfetal and may coincide with the organization of oral, anal and genital patterns of behavior.
Winkelmann, RK. (1959). "Erogenous zones: their nerve supply and significance". Mayo Clin Proc. 34 (2): 39–47.