Sorry about the long post, I have quite a few experiences I'm eager to share lol.
Here's my top 4, based on how much the instantaneous pain hurt:
#4: Meningitis with a white blood cell count of over 12,400. IIRC, 1-2 is considered normal, 5 means you're sick, and anything over 20 can be very dangerous. Many people with a count in the thousands die within hours, but I was very lucky to be treated in time. Over the course of my 10 day stay in the hospital, I: lost consciousness for 18 hours while I was "combative"; had 2 spinal taps; had 2 catheters put in me (the first attempt wasn't pretty); had about 15 IV's put in my arms because they kept falling out or filling with blood (and I have a phobia of blood and needles, lol); and had my first herpes breakout while immunocompromised, which nearly caused fatal encephalitis. It was definitely the #1 worst time of my life, at least.
#3: Ear infection I had for a week while my family was camping. The only medication I had were some ear drops that my doctor later told me had probably only made the pain worse. It was severe enough that I couldn't sleep for 3 days, and I broke down crying and flew into rages several times before we got to a doctor who could help me. It went away quickly after he administered some medicine via a "wick" he stuck into my ear.
#2: Accidental dural tear during a routine epidural steroid injection (to manage my chronic back pain). Not only was it roughly the equivalent of a surprise spinal tap, but the needle hit a nerve too, so I had to fight to keep my legs from spasming up to my chest and causing further problems with the procedure. The doctor quickly administered more painkillers to relieve it, but the minute of nerve pain down my legs and pressure in my back were the worst pain I had been in at the time...
#1: ...until I got the "thunderclap headache" 15 minutes later. While my dad drove me home, I suddenly felt an unfathomably bad pain in my head (like even I literally cannot imagine how severe it was anymore) and could not stop screaming for 3 minutes. Apparently I said "my head is exploding!", and among other things that makes my doctors and I think that the injected dye and/or the CSF leak that followed caused a big pressure fluctuation around my brain, almost as severe as a brain hemorrhage. I seriously rank that pain as a 12 or 13 on a "pain scale from 1 to 10" so I don't have to underrate everything else. The worst part? I got the meningitis 3 days later, from either a bacterial infection or an allergic reaction to the injection. And I had a chronic CSF leak and then hypersensitivity to pressure fluctuations for 2 years, because of the unhealed tear in my dura.
In my experience, pain never becomes any easier to endure. Fortunately for me, I've been relatively healthy for the last year or so. I hope everyone else here can avoid pain as much as possible in the future