Technically there isn't one.
You want dGhost for HoO Mobs + I think dPoison for HoO Bosses.
You'd also want Either dHoly or dSorcery for CoU. But for some reason oFire seems to work really good against the spiders.
so on so forth. In the end, you're probly going to have 2 armor sets and 3 weapons if you want to [min max] your stuff. But ultimately just using a good 'spread' of elements from your equipment + skills [+ammo if you use it] will enable you plenty of damage and defenses.