I've had a couple of black eyes, concussions, sprained fingers, tendonitis in the elbow, and a REALLY nasty sprained ankle that bugs me to this day...
But if I was going to pick one thing, it would be when I got a flat tire (rear) and decided that instead of walking to my car I would do a hang 5 down the hill in front of the Washington Monument. The hill isn't that steep, but it was long and I didn't touch my brakes. It is also just a sidewalk - one of the really rough blacktop sidewalks.
I got down the hill and was probably going 20MPH or so when I flipped forward. My hands went down and mostly protected my face as I arched my back... But, my hands were pulled under my body. It ripped the skin off the back of both arms and the top of my legs and the back of my hands. Ripped the face off the watch I was wearing... That ranks way up there for pain factor for me. But, in a couple of weeks it was all healed and didn't bug me the way my sprained ankle still does 8 year later.