Plague 6 (boils). The first time a plague affected the body directly. Was so bad the magicians may have been in too much pain, felt unceremoniously dirty, and/or were too ashamed to stand before Moses (who represents God).
7th plague (hail). God reveals His purpose (to show who He is). There is a missionary reason behind it. To know God is to know His power, His sovereignity. He commands the water, animals, heavens, and the whole world (versus Egyptian gods that have "power" over a specific area/thing).