Answer:
It can vary from person to person, but the big ones that should be true to all guys would be; behind the ears, neck, inner thigh, and of course the groin.
That being said, there are some considerations to take into account. If a guy is highly ticklish (as I am) any of these can be as much of a turn off as they are a turn on. This takes some learning to determine what does or does not work. Just using a little more or less pressure may be enough to be erogenous instead of ticklish.