Birth, without a shadow of a doubt. And pain relief doesn't really factor into it - the question asked which hurts more, not which hurts more if the other comes with an epidural.
I got kicked in the bollocks once - oddly, by the bipod on one of my guns. My mother had asked me to show it to one of her friends, and I was demonstrating how to unfold the bipod. Foolishly, I had the gun at the low ready when I unfolded these two 8" spring-loaded steel legs, and they both whacked me where it hurts.
Thing is, yes, it hurt - but certainly not the "automatic reset button" I'd been led to expect. I didn't show anything outward, I just put the gun down and (rather quietly, to be fair) said "Would you excuse me a minute?"
Then I went to lie down in a dark room for a while. But it certainly wasn't debilitating, it didn't turn me falsetto, and it could be ignored for long enough to deal with in private.