No world bosses!! (keeping it a little realistic)
The below is the Bis of the Bis, universally accepted, diligently theory-crafted, Xashe approved.
With the release of BWL, and some +spell hit gear, we finally have a sturdy base to build on. I've started a list of the primo BiS from now till the end of vanilla, broken up into the upcoming patches.
Current Patch 1.6 (BWL)
Helm - Mish'undare, Circlet of the Mind Flayer - Nef
Neck - Choker of the Firelord - Rag
Shoulder - Mantle of the Blackwing Cabal - Razorgore
Back - Cloak of the Brood Lord - Nef
Chest - Robe of Volatile Power - Luci/Shazz/Geh/Sulf
Bracer - Bracers of Arcane Accuracy - Broodlord Lasher
Hands - Ebony Flame Gloves - Ebonroc
Belt - Mana Igniting Cord - Magmadar/Geddon/Garr/Golemagg
Legs - Flarecore Leggings - Crafted
Boots - +30 Boots of Shadow Wrath - BoE
Ring1 - Band of Forced Concentration - Ebonroc
Ring2 - Band of Dark Dominion - BWL trash
Trinket1* - Briarwood Reed - Jed or Eye of the Beast - Quest
Trinket2 - Neltharion's Tear - Nef
2H - Staff of the Shadow Flame - Nef
Wand - +23 Lunar Wand of Shadow Wrath -BoE
*ToEP goes here, but priority Mages > Warlocks, also depending on buffs and raid DPS Reed vs Eye
Patch 1.7 (ZG) changes:
Back - Cloak of the Brood Lord -> Cloak of Consumption - HakKar
Chest - Robe of Volatile Power -> Bloodvine Vest - Crafted
Legs - Flarecore Leggings -> Bloodvine Leggings - Crafted
Boots - +30 Boots of Shadow Wrath -> Bloodvine Boots - Crafted
Patch 1.8 (Dragons of Nightmare/Abyssal Council) changes:
Bracer - XXX -> Rockfury Bracers - Quest
Patch 1.9 (AQ 20/40) changes:
Head - XXX -> Doomcaller's Circlet - Quest
Neck - XXX -> Amulet of Vek'nilash - Twin Emps
Shoulder - XXX -> Doomcaller's Mantle - Quest
Back - XXX -> Cloak of the Devoured - C'Thun
Hands - XXX -> Dark Storm Gauntlets - C'Thun
Belt - XXX -> Eyestalk Waist Cord - C'Thun
Ring1 - XXX -> Ring of the Fallen God - C'Thun
Ring2 - XXX -> Ring of Unspoken Names - Quest
2h / 1h+OH - XXX -> Sharpened Silithid Femur + Royal Scepter of Vek'lor - Viscidus + Twin Emps
Patch 1.10 (non-content) changes:
-Nothing, cry more noobs.
Patch 1.11 (Naxx - non-Frost Wing loot) changes:
Helm - XXX -> Plagueheart Circlet - Quest
Shoulder - XXX ->Rime Covered Mantle - Gluth
Chest - XXX -> Plagueheart Robe - Quest
Legs - XXX -> Leggings of Polarity - Thaddius
Boots - XXX -> Plagueheart Sandals - Quest
Ring2 - XXX -> Seal of the Damned - 4 Horsemen
2h / 1h+OH - XXX -> Wraith Blade - Maexxna
Wand - XXX -> Wand of Fates - Patchwerk
The 2/8 set bonus (Your Shadow Bolts now have a chance to heal you for 270 to 331) is always reached since Tier 3 head, chest and boots are best in slot items. The 4/8 bonus (Increases damage caused by your Corruption by 12%) is very easily reachable with Plagueheart Shoulderpads, meaning that the set bonus worth discussing is the 6/8 bonus (Your spell critical hits generate 25% less threat. In addition, Corruption, Immolate, Curse of Agony, and Siphon Life generate 25% less threat), extremely handy in threat-sensitive fights.
The 6/8 T3 set bonus is most effectively reached by adopting the Plagueheart Belt and Plagueheart Gloves over your Eyestalk Waist Cord and Dark Storm Gauntlets.