A full set of elementium armor might indeed be quite good for PvP, especially when combined with an elementium sword for increased pixie damage in addition to the random hit effect (blindness, wither, slowness, weakness) from the helmet. However, elementium only provides about as much protection as iron, albeit at a much higher enchantability.
Terrasteel armor has the same protective values as netherite, but higher knockback resistance (100% in total instead of just 40%) and optionally the various critical hit bonuses from adding the will to the helmet: healing you by 25% of damage dealt, increasing critical hit damage multiplier as your health gets lower, completely ignoring the target's armor damage reduction (though not any protection enchantments), or applying wither/slowness/weakness.
Also, all Botania armor items can be made invisible on the wearer by applying phantom ink to them.
I don't know the other two proposed armor types, so I can't really provide a comparison.