What is the most painful thing you have ever experienced?

Mine was a cracked rib while having the flu.

Hmm. Would have to be a renal x-ray I had following surgery to remove a tumour. It was 10 days post-op. The technician injected me with the dye, then fastened a strap across my fresh (clips removed the previous day) scar, tilted the table and left the room. He didn't come back for 10 minutes, by which time tears were raining down my face ...

Wisdom teeth extraction in the old days (and remember I had a 9 pound 15 ounce baby without painkillers).

I have a pretty high pain threshold but an impacted eardrum done for me.

On a regular (but temporary), basis, leg/foot cramps are worthy of mention.
I find swearing loudly and repeatedly, while hopping about the room with a limb at an odd angle, eventually scares them away.

Skull fracture.

Can't think of too much. Probably just the time I smashed my finger in a barn door hard enough to eventually have the nail turn purple and fall off. That throbbed pretty good for a few days until the nail finally loosened enough for all the fluid building up behind it to escape and relieve the pressure. I should have just popped it with a needle, but was stubbornly hoping I wouldn't lose the nail. I sprained my ankle once, and that throbbed pretty badly for a couple of days until the swelling started to dissipate (though didn't hurt at all the very first day...I had such a release of natural endorphins that I actually felt drunk with no meds at all!)

Nothing else I've done has been any worse than just smarting for a few minutes and leaving a bruise behind. I've been pretty lucky about not winding up with any serious injuries in spite of so much time around barns.