Ever fell mouth first into a sidewalk? I mean, really hard. "Break a tooth in half" hard. If you did, you'll understand my choice.
I've suffered a lot of physical injuries in my life: I've take rather severe falls into concrete, rocks and, my personal favorite, a pile of cacti and poison ivy. I've nearly impaled myself (through the stomach... not "traditional" impalement, no) on a bicycle handle, broke my head twice, got a nail through my foot, I've had headaches that felt like my brain was being pulled through my spine, and others where it felt like my entire head wanted to explode, headaches that temporarily blinded me, I've been electrocuted, burned, drowned, stabbed, yatta, yatta, yatta... Bottom line, I've "tasted" a lot of different injuries, a LOT, and still, that was, by far the worse physical pain I've ever suffered.
It's really hard to describe, but it was like an electrical surge starting at my tooth and going through my entire body, probably the most intense pain I've ever felt. Followed by a second wave of just pure agony. It was just sickening.
The only other injury that I could even match to it was also one of the least spectacular: Skateboarding. Fell during a jump on flat terrain, landed perfectly flat on the ground, like a wooden board. The pain was so intense I couldn't move for about 10 minutes. The worse part there was the honest feeling that I was suddenly paralyzed. That one was particularly hard to describe because it was just... pain. Absolutely generic kind of pain, when you hit your body somewhere. Except ALL over, and so much of it it overflowed every other sense.
By comparison to those two, both times I broke my skull I woke up on the floor laughing and drinking my own blood... So, yeah, those two were rather vicious.
Drowning sucks too, but mostly due to the psychological desperation it causes, not so much for the actual physical pain.
