I suggest accuracy and recharge IO bonuses. Twilight Grasp and Fearsome Stare need accuracy and Tar Patch, Howling Twilight, and Dark Servant can use the recharge.
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.
I got a dark,dark,dark, def at lv 50 and i six slotted my: Dark Blast,Gloom w/set ench Devastation. Tenebrous Tenacles,Night Fall w/set ench Positron Blast. Moon w/set 6 ench sting of the mant. Health w/set 6 numi. Twilght w/5 slots of Numi and a IO debuff. TarPatch 3 slot pace of the turt. Stamina 6 slotted set perf shifter. Darkest Night 3 slot set Dark Watcher. Shadow Fall 3 slot set LotG. Fearsome Stare 4 sloted set GotA.Dark Serv 6 sloted ether unbreakable constraint,lockdown or Enfeebled Operation. Dark Consumption 3 slotted Perf Shifter. Soul Drain 3 slotted Fire Gaussians. Soul Transfer 3 slotted ether IO recharges,Numi, or Perf Shifter. Now some things u can play around with to suit your personal playing styles but thats for the most part how I've set up my D3 Def (damage, more of a solo Def) or you can put more Dark Watcher sets if you want to debuff more and for teaming (More of a team Def). Hope that helps you pick set enhancements out 
Not a fan of grammar, spellcheck or proper sentences.But will have fun anyway
Don't forget that Lysozymes are a really nice way to put in acc/-tohit. They usually aren't too expensive, and can usually be traded for at the end of a Statesman TF :-) I used two to enhance twilight grasp (with 2 miracle and 2 numina - both heal, heal/recharge), 3 in Fluffy (cause that's all he needs :-) and I threw in one and an enzyme (end/-tohit) in Fearsome stare with 4 GotA. Also, if you have extra slots the centriole (dam/range) is VERY cheap and does wonders in the two cones (nightfall, tenebrous). While they don't give you an excess of set bonuses, sometimes the power is more important...
I'm leveling up my dark/dark defender, and am getting ready to begin slotting sets. I'm curious as to what is as to which set bonuses would be most useful to go for? Recharge, defense, accuracy? Any particular sets that are a must to slot in certain powers?
Goodbye, City of Heroes...I'll miss you.
On Justice as @Amperrie (well, for a little while longer...)