That depends. Assuming you mean natural childbirth but even that varies from person to person and birth to birth.
My first child was by far the most painful. I had absolutely zero pain relief, and it was agonising. I'd put that at a 10, but I also had the drip to help things along and that makes contractions much more painful.
Second child was very painful but a lot faster, I would put him about 8-9. Gas & air with him.
Third was a total breeze. I was able to talk through all my contractions even through established labour, right until I had to push. I wouldn't put her more than a 4-5 at the absolute most. She was also an hour longer than my second. A few puffs of gas & air right at the end.
Fourth was very painful but very quick, I was in labour in hospital 1 hour 55 minutes. Had gas & air but also the birthing pool. That was an absolute godsend. I'd put him at about 7-8.
Fifth I was induced (water broken) and was in what felt the same as established labour for a good six or more hours before they said I still hadn't got past 2-3cm... then had to have the drip at which point for the first time ever I told them I needed an epidural. Never felt the need for it before her. Probably more panic than anything else but wow, the relief was unbelievable. The pain was about 7-8 before it, I knew the drip would make it worse. With the epidural the pain gradually vanished. The thing I felt when she decided, 45 ish minutes later to be born, was huge pressure and I found that frightening. But it wasn't the same pain though. I can't really put a number on that because it wasn't pain as such, and it literally lasted a couple of pushes.