I thought I posted to this already but I guess not.
My worst was from a pruning bow saw. I was young and cutting some branches from a sapling. I was cutting with the sapling standing upright instead of laying down (first mistake) and my hand underneath the branch (second mistake). Once through the branch, it kept cutting clean through my thumb. It split my thumb in half right through the nail all the way down to the nail bed. It was a nasty, jagged cut and bled so bad, I had a river of blood running off of my elbow. This one hurt as bad as anything I ever did to damage myself.
My second worst was not from a knife, but it was a sharp edge. When I was a kid, my parents had an old Ford sedan. We were packing up to leave after a trip and I was outside playing around the trunk. I ran my hand around the inside of the weather seal and somehow got my left index finger between the seal and the metal lip. Well, the lip wasn't rolled, it was sharp - very sharp. I sliced the end of that finger almost clean off. It bled like crazy and I got taken to a doc in the box. He should have put a stitch or two in it but since he didn't, it sort of bunched up and now I have this little bump of scar tissue under the surface. I CONSTANTLY "finger" it, sort of like when you get a burn on the top of your mouth and you can't stop messing with it.
My wife just recently stabbed herself in the palm of her hand while trying to cut something or other in the kitchen. She had to get several stiches and it bled really bad.
My dad has the worst though. He basically raised himself and when he was young (like 5 or 6), he was reaching up on the counter for some food or something and accidentally grabbed a large cleaver. It fell and cut him in the arm very badly. It left a scar which wraps around his entire forearm and all these years later it's very visible.