What is the best over-the-counter medicine for rash under the breasts?

Depends. If you have large breasts and the area under them is moist, then using antiperspirant powder on the area may help. If the area is read and "rashy", try an anti-fungal cream. If the rash is itchy and dry , then it may be from a contact allergen. In the latter case , Hydrocortisone cream may help.