My worst pain is either biting my tongue off, or a fun trip to a hospital in a coma. They rank for different reasons.
The actual biting off of the tongue wasn't so bad, I actually hadn't noticed until a few moments later when I tried to talk and ended up just spraying blood and flesh. Then I had a bumpy ride to the hospital, a 30 min wait then they sewed it back on without anesthetic. It hurt a lot, but the worst was the next two weeks. Due to my tongue stitches, I could not eat solid food, and had to rinse my stitches with concentrated salt water after every meal or snack. I hate baby food. To this day, I cannot taste certain things, mainly bitter.
The going into the coma bit was relatively painless, OD to try to kill self, walk off into woods. The coming out was hell. The stuff I had OD on caused muscle spasms as a regular side effect. Thus I spent the first two days in the hospital unconscious, strapped to a bed after inadvertently punching two nurses and my dad. Upon gaining consciousness, I found I had a catheter installed. Imagine a pen shoved up your urethra, and it scraping against the inside of your penis every time you move, and I had muscle spasms. Despite the pain it caused coming out, it was worth being free.