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.