Nipple piercings aren't painful for everyone, some people barely feel them
it's probably more painful if your nipples are sensitive, mine aren't, they probably wouldn't be too bad.
There's some extreme piercings, deep ones that are pretty painful but I don't know which would be the MOST, depends on the experience of the piercer and the person themselves!
I do hear that cartilage piercings are pretty painful, but I doubt they're the worst. I've consistently heard that targus piercings are awful, that's probably the worst of the common ones.