Here is quick order of buffs and debuffs according to a basic set up (bard/chon). Keep in mind there are fights where combat buffs will be used in a different order. It's very important we keep the proper buffs and debuffs on throughout a fight so we can pull higher dps.
If anything has been removed with new expansion please state in comment below :). Again this is a reference guide.
Debuffing: These should be used and kept up the entire fight with the exception of power core/wild growth.
Archon - Pillaging Stone, Ashen Defense, Crumbling Resistance, Lingering Dust
Bard - Coda of Jeopardy
Inquis - Clinging Spirit (Bolt of Depravity/Soul Drain)
Chloro - Wild Growth
Tact - Power Core
Marksmen - Lightening Fury
Assassin - Lethal Poison
Pillaging Stone (Mage/Archon)
Increase Damage Taken - These do not stack. If you have a tact + Chloro make sure they do not overwrite.
Wild Growth (Mage/Chloromancer) lasts 12s, requires charge
Power Core (Rogue/Tactician) lasts 15s, +10% Damage Taken
Increased Physical Damage Taken - Archon/Oracle priority. Bards should not use Cowardice
Ashen Defense (Mage/Archon)
Coda of Cowardice (Rogue/Bard) requires combo points, with 2/2 Extended Grief
Curse of Frailty (Cleric/Oracle)
Increased non physical damage taken 1 - Archon/Oracle priority. Bards/Inquis should not cast their debuffs.
Crumbling Resistance (Mage/Archon)
Coda of Distress (Rogue/Bard) requires combo points, with 2/2 Extended Grief
Spiritual Deficiency (Cleric/Inquisitor)
Curse of Consumption (Cleric/Oracle)
Increased non physical damage taken 1 - Tough one. Inquis is stronger but defiler will overwrite it (and vice versa).
Clinging Spirit (Cleric/Inquisitor) applied by Bolt of Depravity or Soul Drain
Sigil of Spirits (Cleric/Cabalist)
Tenebrious Distortion (Cleric/Defiler) applied by Marrow Harvest, spread by Unholy Nexus
Embers Bomb (Rogue/Saboteur)
Slagged (Primalist/Vulcanist) applied by Conflagrate, spread with Skill Shot through Totemic Power mastery
Damage Proc - Bard priority. Lasts 60 seconds.
Spotter's Call (Warrior/Beastmaster) with 2/2 Hunt Master
Coda of Jeopardy (Rogue/Bard) requires combo points, with 2/2 Extended Grief
Thorns of Ire (Cleric/Druid)
Corroded Defense (Cleric/Oracle)
Armor Debuff - MM's should keep this on. The other roles are not widely used.
Mark of Inevitability (Warrior/Champion)
Lightning Fury (Rogue/Marksman)
Piercing Shot (Rogue/Ranger)
Splinter Bomb (Rogue/Saboteur)
Decaying Defenses (Warrior/Reaver) applied by Flesh Rot, spread by Plague Bringer
Chance to be Crit - Assassin takes priority because Dervish has a pull to with it.
Mark of Extermination (Warrior/Champion)
Lethal Poison (Rogue/Assassin) with 5/5 Poison Mastery
Dauntless Strike (Rogue/Bladedancer) requires combo points
Damage Debuff - Situational. Use during big damage events such as Yrls Maw or Izkinra's breath attack
Power Drain (Mage/Archon) toggled, requires charge
Waning Power (Mage/Archon) -5% AP/SP, ST, lasts 15s, 1m cooldown
AP/SP debuff 2
Rift Disturbance (Rogue/Riftstalker)
Cast Speed debuff - Archon
Lingering Dust (Mage/Archon)
Hit Chance Debuff
Wrath of the Planes (Rogue/Riftstalker) requires combo points
Healing received debuff
Septic Wounds (Warrior/Reaver) -15% Healing Received applied by Necrotic Wounds, spread by Plague Bringer
Healing Received debuff 2
Numbing Cold (Cleric/Shaman)
Bond of Corruption (Cleric/Defiler) spread by Unholy Nexus
Essence Link (Mage/Necromancer)
Fell Blades (Rogue/Nightblade)
Vampiric Munitions (Rogue/Marksman)
Dire Wolf's Maul (Rogue/Ranger) -10% Healing Taken with 2/2 Master Huntsman
Healing Done debuff
Vile Affinity (Warrior/Reaver) -15% Healing Done applied by Soul Sickness, spread by Plague Bringer
Ability Cost Debuff
Thank you Raki, Destined