[Taqwus]

Belial might be more effective than a Succubus. There's also no thematic reason why he'd be limited from corrupting female commanders, as well, and he probably has a far better chance of surviving a failed corruption. He also generates free blood slaves and meat shields, IIRC. Perhaps still not a particularly efficient purchase, but...

Now, if you get the Plague Wind, that might be a problem; doesn't he spread disease to the province he's in? Hm.

If you like thematic SCs, the Unquenched Sword seems obvious for any of the Archdevils, as would the non-unique flaming swords, demon whips, and perhaps tridents (stretching this a bit). For the Archdevils and Ice Devils, don't forget the MR boosters (starshine skullcap, antimagic talisman are cheap), flight/quickness (boots of quickness or Jade Armor for the Archdevil, winged shoes for the Ice Devil and have the ID script Quickness), perhaps a luck shield if you're using a one-handed weapon. And if you're dealing with specific attack forms, e.g. massed Incinerate and you've got an ID, get the relevant immunity (unless impossible, e.g. Drain Life is not really blockable; you can only mitigate it by massive reinvigoration e.g. Boots of Calius, regeneration, life draining attack of your own, other units to keep fighting while your unconscious or to absorb Drain Life spamming).