I have fairly constant back and neck pain, which I'm pretty sure has to do with the amount of sitting in front of a computer I do. However, that's sort of a constant and unremarkable thing.
The worst sort of pain I can think of actually occurred to me last night. Stomach flu, basically. Constant vomiting to the point of spewing stomach acid out from my nose, diarrhea, and total body pain due to being unable to sleep or even lie down in one position. Constant spasms and shivering due to fever. I know it's something everyone gets occasionally (I don't get sick often at all), but it's truly hellish and completely debilitating. All you can really do is wait for it to run its course, though, so you're left to experience every agonising minute of it in complete detail.