I think Elemental Resistance is the most widely useful(The Elemental Resistance+Sanctity Plate combo on Vhala is really impressive, the givini ruins would have been a bitch if vhala took full damage from the final boss's attacks when I cleared it)
Debuffs are rare and you can always use buff skills to counter them.
So the only question is armour type. Physical makes it available to more characters and Uyae is the best candidate to use it full time, but spellcasters are squishy as fuck and halving elemental damage is more useful for them than anyone else. I don't recommend the sexual armour option though. Riala Altina and Yarra can wear the magical version as well, and Qum and Nalili aren't especially high priority choices for wearing it.