[Vicious Love]

No point in giving a succubus a weapon, it'll lose it's lifedrain attack. I usually equip them with a mix of items that capitalizes on their high defense skill. Boots of Quickness are ideal, as is a Cat Charm.
A Lucky Coin is a must, unless I feel I can afford a better shield(Ideally a Shield of the Accursed. Goes well with the aforementioned defense skill, AND makes sure that anything that can survive a succubus attack won't survive the horrors you send to avenge her death), or feel compelled to hoard astral pearls, and give her a Main Gauche of Parrying instead.
Since the succubus is an assassin, some sort of summoning item is a must. The Skull Talisman is pathetic, considering how much more effective simply sending the succubus into hand-to-hand is. The Skullface isn't much better, and isn't cost effective.
The Bottle of Living Water is OK, but the only proper way to use it is to put your succubus at the front of the deployment grid, within reach of mounted assassination targets. Otherwise, enemy mages will have a few turns to prepare before the elemental reaches them, and flying bodyguards will have a few turns to attack your succubus.
By simple process of elimination, we conclude that my favorite goon-summoning succubus item is Lifelong Protection. The imps can kill plenty of targets without the succubus putting herself at risk, and can fatigue or injure others before the succubus administers the coup de grace. Best of all, their mob tactics usually prevent mages from bringing any spells to bear. I find that sticking a Protected succubus near the back of the deployment grid(Possibly with a Black Bow of Botulf or one'a them vicious crossbows), with hold orders for 3 turns or so, followed by "attack rearmost", takes care of most AI commanders, certain pretenders included.
If you must give your succubi headgear, I've found you can't go wrong with the Horned Helmet(Especially along with Boots of Quickness), for that little added oomph. The -1 defense penalty is a tad harsh, though. I suppose if you and your succubus have progressed to that stage of the relationship, you could give her a Wraith Crown, just to show her she's special to you. The price is awfully steep, though, and anything that can threaten a well equipped succubus can almost definitely ignore ethereal status.