Cornuthaums are one of the best helmets for a wizard to use, since they can be safely enchanted up to +7, and the clairvoyance property is quite useful and can't be permanently gained via any other method besides the Amulet of Yendor itself. Although a very powerful helm, cornuthaum is not worth wishing for by powerful wizards. If you can polymorph items, consider polypiling instead.