I love breast stimulation. However for a while when my kids were nursing, I didn't want My Man to touch them. I just felt they were not only "utilitarian" but they were oddly sensitive and I didn't want to have him where the baby was most of the time.
When my babies weaned, the sexual sensitivity in my breasts returned, but it took a while, like years. *sigh*
As for the "most sensitive part" I think the clit is much more sensitive than my breasts, my vagina is more sensitive than my breasts and my butt is more sensitive but every woman is different.