Hmm, I'm not sure of pain but I have a story that the question made me think of. It's more of what is the worse thing you have ever tasted? I don't mean to derail your question but it did make me think of this. One time I had my jaw broken, 'cause someone punched me in the jaw.
The fix was to screw these metal screws through my gums and into my jaw at the front of my mouth, two on the top and two on the bottom. They'd put these rubber bands on them to hold my jaw shut while it healed.
You couldn't really open your mouth more than a couple of millimeters. So pretty much for a while I was limited to liquids. I had a lot of protein laced milk and Metamucil and occasionally I liked a McDonald's thickshake for a treat. I was so desperate I even tried the strawberry one, which at the time was amazing, but since I have never had again, which says something.
Anyway, with all these liquids you needed to clean your mouth, you needed to clean your teeth. I was religious about this, normally I floss and brush twice a day, so not being able to had my skin crawling. So after every milk drink I'd rinse with water, rinse with salt water and then rinse with Listerine and then with water again. There were stitches all in my mouth as well as the screws through my gums so they told me to keep clean.
A few weeks later when I finally got the elastics off the surgeon asked me to open my mouth, I could only open about an inch. He peered inside with a torch and said "Ahh!", swiveling off on his (undoubtedly expensive) chair off to his desk.
He came back with a paddle pop stick, like a tongue depressor, and leered into my mouth. I felt it rest against my gums and heard a nice 'sssssssss' over his joyful "oooo!" I heard the hiss in my skull, I remember gargling "WHAT WAS THAT?" and then I tasted it.
My mouth was full of this warm, salty, foul taste of death. "Ripe, salted off" is the best way to the describe it. It was like fruit that you'd forgotten in the crisper drawer of your fridge. The surgeon asked me to spit in his sink and I spat out half a shots worth of yellow puss, swirled with blood. It would have made a pretty cool marble in primary school.
After all that religious mouth washing, I still ended up with an infection. I was happy though 'cause I had the bands off. I went home and tried to eat shredded chicken and felt like crying when I couldn't fit it through my gap.