Magic resistance

Magic resistance is the number one extrinsic to get. It will protect you from magic missile attacks, several types of monster spells, death magic such as from a wand of death, any sort of unintentional polymorph, teleport traps, and it will reduce the number of objects cursed by a malignant aura. For all that, it's monstrously difficult to get. Only cloaks of magic resistance, gray dragon scale mail, Magicbane, and a few Quest artifacts will grant it. In addition, relying only on your own Quest artifact for magic resistance is dangerous in the endgame, because the Wizard of Yendor can steal it from you and then hit you with Touch of death; a Quest artifact of another role, obtainable only by wishing, is safe from this kind of theft (but not from that of nymphs, of course).

Quest artifacts granting magic resistance when carried
Role	Artifact	Alignment
Archeologist	the Orb of Detection	Lawful
Caveman	the Sceptre of Might	Lawful
Knight	the Magic Mirror of Merlin	Lawful
Monk	the Eyes of the Overworld	Neutral
Tourist	the Platinum Yendorian Express Card	Neutral
Wizard	the Eye of the Aethiopica	Neutral