What's the best treatment for boils on the thighs?

Boils. These are, more often that not, due to staph aureus infection, and you should be seen by a doctor, have one of these opened and cultured. The important factor is to determine whether or not this is mrsa, and if so this will influence the choice of antibiotic. You might also need to attempt nasopharyngeal and skin colonization. Good luck.
See a dermatologist. Hidradenitis suppuritiva is possible. You probably chould get a prescription.