What combo has the most -damage powers?
Ice Armor/Kinetic Melee Tanker; Anything that can use Kinetics (the buff/debuff set).
My guides:Dark Melee/Dark Armor/Soul Mastery, Illusion Control/Kinetics/Primal Forces Mastery, Electric Armor
"Dark Armor is a complete waste as a tanking set."

Kinetics will be able to debuff any one target down to about 25-50% damage pre FS, and Post FS will floor 1 enemy and get the rest at 50-75%.
Ice/kin/soul tanks can get single target to about 15% (if they take the right interface, can't think of the name) and they can get aoe about 60% IIRC.
That means total damage, not debuffed damage. So 60% in my last figure is -40% dmg.
Kin/DP Defender using Chemical Ammo.
While not quite up to Kinetics defender levels:
Cold's Benumb or Thermal's Heat Exhaustion are also note worthy (62.5% single target)
A KM brute adding in Darkest Night to the mix.
And then there's Defender Dark Miasma with Twilight's Grasp (stackable far as I can tell) plus Darkest Night. So Cold, Thermal and Dark Miasma Defenders paired with DP chem ammo (in addition to Kin/DP).
I second the dark/dp defender combo. Kin and colds are pretty uber, but with a smudge just under -40% dmg aoe with darkest night running, dark is more reliable for my purposes.
Hmmm, Time manipulation/dp chem would be my suggestion. Partly because I can control better where time juncture is than where darkest night is.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
Kinetics can stack 2 Fulcrums + 3 Siphon Powers on a single target, to essentially reach the -damage cap (or floor, depending how you look at it).
Dark Miasma can stack Darkest Night, 2-3 Twilight Grasps, Dark Servant's Darkest Night, and Dark Servant's Twilight Grasp to get similar results.
Those would be my choices for a single target (and part of why I love them for AV fights). For AoE they probably win out too, but don't reach as impressive levels. Other buff/debuff sets can have more potent individual -damage powers, but don't have the same potential to stack up to the cap. I'm not so familiar with melee ATs though, so will defer to others on Ice/Kin tanks and such.
As for reducing -damage, relative enemy level will reduce it (as it does with all powers - damage, debuff, or mez). Also damage resistance will reduce its effects on any attacks that match the damage resistance's damage type (so a mob with smashing resistance will be less affected by -damage on its smashing attacks).
I'm curious to know which powerset combos can dish out the most -damage debuff. Does enemy rank or level reduce the effects of -damage?
Iron Crane - Inv/SS Tanker (Champion)
Fox Die - EM/Nin Stalker (Infinity)
Azure Hunter - Arc/Ene Blaster (Infinity)
@tyxavier
"Consider the unparalleled advantage, of a natural disaster thats impossible to manage."