What are the best modfiers for the good items on terraria?

There were no new modifiers added in 1.2, so the modifiers for these items are the same as the ones for their respective classes.

The best modifiers are:

• Godly, for melee weapons that cannot have their speed modified, or for melee weapons that cannot have their size modified. (That means melee weapons that are not swung.)
• Legendary, for other melee weapons
• Demonic, for ranged weapons that have no knockback.
• Unreal, for other ranged weapons.
• Mythical, for magic weapons with knockback.

For magic weapons without knockback, you can choose either Mystic for reduced mana cost, or Demonic for increased damage; there is no clearly better option.

The Paladin's Hammer is a boomerang. Although it is ranged, it does melee damage. Since its speed/size cannot be modified, its best prefix is Godly.

The Vampire Knives, like boomerangs, is a ranged weapon that deals melee damage. Its speed and size also cannot be modified, so its best prefix is Godly.

The Golem Fist looks like it's similar to the Harpoon, in which case it's a ranged weapon with melee damage. Its best prefix is, again, Godly.

The Magical Harp cannot have knockback, so its best modifier is Mystic or Demonic, depending on how you like to play.

_{Note: It seems like anything that does not consume ammo deals either melee damage or magic damage, even if it is a ranged weapon.}