Does cortisone, cortaid, (hydrocortisone) or lotrinim work best on getting rid of a armpit rash?

Depends: If armpit rash is caused by allergic/irritant contact, cortisone might help. If rash is yeast infection or caused by fungus, lotrimin (clotrimazole) would help. You may need to see a dermatologist to get idea what cause it, because wrong medication may make it worse. For example, if the rash is a fungal infection and you use cortisone, it would make it worse and spreading.