What is the strongest OTC pain medicine to take?

None. There is no good answer. The choice of medication is dependent upon cause, type, severity of pain. I personally do not feel it is a good idea to take otc pain meds except for minor problems such as menstrual cramps and common mild headaches. If you are requiring frequent otc pain meds, find out what is wrong.
Ibuprofen. 4 otc Ibuprofen pills equals 800 milligrams. This is available as a prescription as well. You should not take more than 2400 mg in 24 hours as it can cause problems with your kidneys.