For a DC caster you have three primary things to worry about: Casting stat, spell focus, and spell penetration. Ideally all three are maxed, then fit in whatever else you can. For a DC cleric I care about necromancy, enchantment and evocation, meaning spell focus mastery would be greatly preferable to individual school bonuses. So let's see:
Wisdom
20 Enhancement (NECKLACE: Fleetfoot Necklace is the only 20 wisdom item in game?)
9 Insightful (ARMOR: Legendary Scales of the Exile is the only insightful wisdom 9 item in game?)
4 Quality (Any crafted slave lord item, or CLOAK: Legendary Earthen Mantle, or RING: Legendary Chieftain)
2 Profane (TRINKET: Epic Litany of the Dead, GLOVES: Bluescale Guides, RING: Legendary Band of Insightful Commands, also armor but armor is scales of exile)
2 Artifact (5-piece slave lords set bonus)
2 Alchemical (SHIELD: Alchemical shield. Also weapon, but nightmother is definite and no point going TWF if offhand is alchemical)
2 Festival (Yellow Augment)
1 Exceptional (Augment; could get +2 from legendary greensteel but that's only +1 over augment and costs -2 alchemical)
Spell Focus Mastery
7 Enhancement (CLUB: Nightmother's Sceptre is only source in game?)
4 Insightful (Nightmother's Sceptre; also found on an ORB: The Legendary Mistfallen)
4 Artifact (Slave Lords 5-piece set bonus)
2 Quality (RING: Legendary Spinneret, also ORB: Legendary Mutilator of Minds and Epic Increased Potential)
1 Profane (Shadowscale armor; conflicts with Insightful Wisdom 9 on scales of the exile)
Spell Penetration
8 Enhancement (Nightmother's Sceptre; also found on other items, but we're using nightmother for spell focus anyway)
4 Insightful (Nightmother's Sceptre)
2 Quality (DAGGER: Legendary Deep Demise, conflicts with Alchemical)
Have I missed any types of bonuses for any of these effects, or any alternative sources for max value for any of the types?
Starting with just the clear best-in-game items listed above, we end up with:
Head:
Eyes:
Neck: Fleetfoot Necklace (Wisdom 20)
Back:
Wrist: slave lords 1
Hand:
Waist: slave lords 2
Feet: slave lords 3
Ring: Legendary Spinneret (Quality Spell Focus 2)
Ring: slave lords 4
Trinket: Epic Litany of the Dead (Profane Wisdom 2)
Armor: Legendary Scales of the Exile (Insightful Wisdom 9)
Mainhand: Nightmother's Scepter (Spell Focus 7, Insightful Spell Focus 4, Spell Pen 8, Insightful Spell Pen 4)
Offhand: Alchemical Shield (Alchemical Wisdom 2)
Thanks to Vish for pointing out that spinneret counts as the fifth slave lord item.
Thanks to silinteresting for pointing out the new club in U39 for a turn undead swap with alchemical shield.
Thanks to cru121 for clarifying that profane well rounded can also go on ring and gloves.
Everything for DC casting is now slotted, plus we have some wiggle room, being able to move profane from trinket to gloves. That means we can move any one of the crafted slave lord items to trinket and still fit everything. That'll make identifying the undisputed best choices for the last four open spots a bit trickier since we can also consider bracers, belts and boots if there is a truly great one.
I'll post a second gearset with the rest of the items filled in. Still looking.