I've never obtained a serious hospital worthy injury but my most painful was...well, very painful, haha.
I was riding round a large field on my 5 year old who had a bad habit of bolting and throwing mean bucks. He was just getting a lot better and hadn't bolted/bucked in that particular field the last couple of times i'd ridden it. We were just walking down the short side of the field, there was a tractor behind a hedge in the field next to us, ploughing. Dougal obviously decided this was a good excuse to go nuts, bolted off at the speed of light and launched into a rodeo-worthy bucking fit. I stayed on for about 3-4 bucks before getting launched over his head and landing on my own head, before flipping over onto my back/side. Holy hell, it was agony. Dougal was gone like the wind, galloping off down the field like a free man. I got up and felt totally out of it, my head didn't even feel like it was attached to my body but eventually started running down the field after him, screaming his name, haha.
I was in so much pain for the next 2 weeks. My back was agony, my groin, hip and right leg were also horrendous. Ouch.
All my other [many!] falls off Dougal haven't resulted in much more than scrapes, bruises and stiffness. He has jammed my leg into fence posts a few times which hasn't been much fun. Brat.