Let me preface this with I have never had anything pierced, and I could answer this, but it would so gross out the entire fluther population and lead to me being permenantly banned, so lets just say J is headed in the right direction.
Nipples. Because of the toughness of the tissue, it’s harder to pierce than genitals. Genitals is just sort of a quick shove of the needle and then it’s over, but nipples it’s not uncommon to hear the phrase “it’s almost half way through now!”
Obviously I’d say your “private parts”, but I’ve also heard that piercing your nose hurts more than any other “visible” piercing.
I’ve had a few tattoos done and laughed my way through them, but I did shed a tear or two over my nose ring from a few years ago.
When asked this question or the same one about tattoos, I always answer with “All of them. All of them are the most painful”. If someone is worried about the pain, they might as well expect the worst. Pain is relative and hard to compare between people. Piercings are quick and when done with a straight needle, the pain is minimal and short lasting. It’s scarier to decide to get the piercing than to actually get it.
For me, it was my tongue piercing. I’ve only had a few piercings though (my ears, my navel, and my tongue). I had my navel done 3 times (first time when I was 16, second time when I was 18 and got home from basic training, third time after my son was born). Even getting it done the second and third time through scar tissue hurt less than my tongue piercing.
In theory, I think nipples would hurt more than genitals because of the tissue, like @papayalily mentioned.
@Adirondackwannabe I read it somewhere, and then my sister verified it from first hand experience (always something you want to hear from your 14 year old baby sister…) and then a couple different tattoo artists agreed a couple years later.