Let's see, What the worst? You decide.
First off, you need to understand, that I don't ride in arenas or fairgrounds. I ride in some pretty rough stuff.
On a pack trip in the San Rafael Swell area. Going into Robbers Roost. We were winding up a narrow trail, A pack horse rushed up and bumped a saddle horse/rider. That horse went over a 40' cliff, Rider baled, but horses crashed and broke it neck. Dead Horse.
And ride in the same area my friend's horse ruptured an aorta ( at least we think.) it was trotting along and suddenly started weaving and become unbalanced. Rider got off and the horse laid down and died, all with in about 5 minutes. We took the saddle, cut the brand off and left.
While riding in Yellowstone Park, a friend had a horse break a leg. We had to cut it throat with a knife because park rules prohibit carrying any guns in the park. Since then we always sneak at least one pistol in. Never want to have to put horse down that way again.
I was riding a young green colt. He was pretty herd bound to the gelding I had my Brother in law riding. We came to an area that I thought looked boggy, So I tried to get him to circle around to the right, My Brother in law went left. The colt was upset and fighting me, lunged into the area ( and it was very wet boggy under the grass) and immediately went dow and started thrashing, I stayed on about 3 bucks and then came off, The horse landed on top of me. I was under his belly and reached up and grabbed his halter and pulled his head down to stop him from thrashing any more. My Brother in law, seeing my legs sticking out from under the horses belly and thinking my head was under the mud, came a running, waving his arms, trying to scare the colt off me. Didn't know I was trying to calm the horse. The colt paniced and started fighting again and got a hoof up on my chest and as it pushed off with that hoof, broke two ribs on me. It was a long painful ride back to the truck 15 miles away and a two hour drive back to home.
I was also riding with some friend in the desert, I was the 4th horse to cross a small stream, I felt my colt struggle a little like the mud was holding his feet. The 5th horse right behind me, went down as it followed into the same spot, in what turned out to be quicksand. His whole front end disappeared. The horse was covered in mud from the cinch forward. His back feet stayed on firm ground and hauches never went in. The rider was flipped over the horses head and landed on his back in the river.
Here we clean the mud out of the horses nostrils and eyes.
While at a CTR, I witnessed another fellows horse slip while crossing a ditch. The horse fell and landed on the rider. He was under water, with the horse upside down on top of him. The ditch was so narrow the horse couldn't roll off. The horse was trapped like a turtle upside down. We all jumped off and grabbed the horses legs ( trying to not get struck by the flying hooves as the horse struggled) and pulled him off the rider. He about drowned. Besides being underwater, the saddle horn had knocked the wind out of him. He pulled from the event. Too sore to ride.