There isn’t one. I say this as someone that always has bags of haberneros around and eats roughly 7 a day including the occasional raw one. They are definitely hot as hell but they also have good flavor. The stuff that I cook for myself is so hot that no one else that I know except for mother can eat it and I am from Louisiana. Occasionally, I will get a hotter than normal batch of haberneros or Scotch Bonnets and I can barely eat it myself. The cashiers at my large suburban supermarket tell me that I am one of the few people that ever buys them.
Yes, they burn badly on the way out as well if you eat enough of them. That is how I taught myself a form of meditation “This too shall pass” to deal with pain. You just learn to ignore it after a while and focus on something else.
I see no point in going higher than that. I have a bottle of Mad Dog Inferno (once one of the hottest hot sauces in the world) that has been around forever and can truly hurt someone. I remember the first time I tried and it wasn’t a fun hour. The only reason I have it is sometimes people come over and brag that they can eat any hot foods. I think, “Oh really, let me start you off with an appetizer then”. A few drops of Mad Dog Inferno and a cracker will dispel all but the most hard-core people of that notion extremely quickly.
The stupid thing is that the hot sauce heat wars have heated up (no pun intended) a lot since even the insanely hot ones were invented. People are just trying to weaponize them now. There is no point to that from a culinary standpoint. It is just pain with little flavor and no purpose.