What is the best modifier for the shield of cthulhu? [closed]

While it will depend on your difficulty, equipment, and what you intend to do, Warding (+4 defense) and Menacing (+4% damage) are popular modifiers for all accessories that can be reforged.

In terms of what the game considers best, based on the effect of the modifier on the item's price, there's a 5-way tie:

• Warding (+4 Defense)
• Lucky (+4% Critical Strike Chance)
• Menacing (+4% Damage)
• Quick (+4% Movement Speed)
• Violent (+4% Melee Speed)

There also exists a +20 Maximum Mana modifier, Arcane, that is considered by the game to be lower value, but it is the best (and only) Maximum Mana modifier.

https://terraria.gamepedia.com/Modifiers