I have a pimple in the stupidest location right now: on the, like, top ridge of my nose, right next to my eyebrow. But while this is an annoyingly central, focal point of a location, I would not classify it as the worst possible place to get a pimple. No, that honor goes to the edge of the lip.
At least in my opinion, that's the worst possible place to get a pimple. It just has to be the most painful--other than maybe the crevice of the nose--and a pimple anywhere near your mouth always risks resembling a cold sore, which will of course prompt the inevitable herpes jokes from friends. A lip pimple is also basically un-coverable, since any concealer you apply will come off should you apply lipstick or lip balm, or you know, eat/drink anything at all. It's also basically un-poppable because doing so is just too excruciatingly uncomfortable.
