Probably lower back going out and laying on the floor unable to move for an hour...couldn't even crawl.
Close second- was on a canoe trip in the middle of nowhere (no cell, no roads, no cities for 50+ miles)- rapid pushed the canoe towards a large rock- I stupidly pushed off the rock with my hand not seeing it was a jagged rock. I split my hand wide open and really deep- I could see tendons or some white shit down there...it was actually kind of fascinating when I was looking at it until I really realized what happened. The actual injury was pretty painless from the adrenaline. Luckily one of the guys on the trip was a surgeon and had a basic medical kit with him, so I had the pleasure of getting a bunch of stitches on the side of the river. That sucked, and the worst part was we were 3 days worth of canoeing before any signs of humanity including some fairly intense rapids, so I had to 'use' the hand as best I could w/ the fresh wound. And of course was told to keep it out of the water - fast forward 3 hours and we flipped the canoe. That hand hurt like a bitch for months, and years later if I hit something on just the right spot I'd get a huge spike of pain. It's finally healed now and the scar blends in w/ the other lines on my hand. I have always wanted to go to a palm reader though to see what they would say the line represents.