Caster equivalent of a Fungi Tunic?

[Gylen]

For melees, a fungi tunic is pretty much the top item to increase leveling power.

What would be the caster equivalent of this kind of item? What would be the top item, for casters, would increase their leveling power?

I thought about Manastone, but a caster class like Enchanters might not have the best use for it.

Your thoughts?