Hero Shield

Alone, this is a defense accessory with the largest flat bonus any single slot can give you, plus knockback immunity. The aggro increase is the part people overlook: it deliberately pulls enemies toward you. That is exactly wrong if you are the fragile one in the party and exactly right if you are the wall.

In a group it does the job it was designed for. While your teammates are above a quarter health, a slice of everything they take gets rerouted into you instead. Pair it with a heavy armor set and it turns a squishy mage or summoner into someone who can stand in the open and keep casting.

The parts that make it also make two other accessories, and playing solo you often want those more. A melee build usually gets more out of the Berserker's Glove for the swing speed and knockback, and the Frozen Shield covers you when your health drops rather than all the time. Keep the Hero Shield for team play and for stacked-defense builds, where every additional point is worth more than the last.