Wanted to compile a list, as someone suggested in the other Dark Sim thread (I've already learned a few new ones from reading through there). Post what you steal, and I'll add it here! Also let me know if any names are wrong -- going by memory for some of this.
-- Raids --
Blackrock Foundry
Iron Flame Binder
- Lavaburst - Trash mobs in the area near the entrance. Moderate damage.
Highmaul
Bladespire Sorcerer
- Molten Bomb - These are the ogres before Kargath. Seems like someone needs to get hit by the spell before you actually get it.
Night-Twisted Shadowsworn
- Chaos Blast - Applies a damage-over-time effect.
Gorian Runemaster
- Rune of Unmaking - Fairly low damage.
Oro
- Radiating Poison - This is one of the enemies you have to kill to spawn Tectus. The debuff does moderate damage, but does not show up as one of your damage sources in the combat log.
-- Dungeons --
Auchindoun (You should glyph Dark Simulacrum here)
Sargerei Cleric
- Void Shell - Not much good reason to let this cast, but possibly a useful defensive for your party.
Sargerei Arbiter
- Arbiter's Hammer - Not recommended to let this cast, as it stuns your tank.
Sargerei Defender
- Avenger's Shield - Not really worth stealing when fighting a Magus or Soulpriest at the same time.
Sargerei Magus
- Arcane Bolt - This is the nuke spammed by the Magus when they are not casting Arcane Bomb. It does a decent amount of damage, but you should instead go for the Soulpriests.
Sargerei Soulpriest
- Shadow Word: Pain - This does a LOT of damage. It's the only stealable spell they cast, with a 8-10 second cooldown.
Soulbinder Nyami
- Mind Spike - Avoid stealing this, does much less damage than Shadow Word: Pain.
- Shadow Word: Pain - Put DS on her while she is casting the first Mind Spike, and then interrupt it. You will get SW:P shortly thereafter. This does a LOT of damage, and you can steal it 3+ times if you have the glyph.
Cackling Pyromaniac
- Felblast - This does a LOT of damage, but it is dangerous to let it cast because it can instant kill your party members.
Durag the Dominator
- Shadow Bolt - Does a decent amount of damage. You should steal this because letting his imp cast Felblast is scary.
Zipteq
- Felblast - This is the imp controlled by Durag. Again, does a lot of damage, but dangerous to let it cast.
Gul'kosh
- Shadow Bolt - Does a decent amount of damage.
- Unstable Affliction - This didn't seem to do too much damage, but maybe the best option.
- Drain Life - Fairly low damage (and channelled).
Grom'tash the Destructor
- Incinerate - This actually does quite a bit of damage, but the Immolate does twice as much.
- Immolate - Does a lot of damage, he usually takes a while to cast it the first time, so you might only get one.
Teron'gor
- Shadow Bolt - Decent damage.
- Corruption - Probably the best steal during phase 1, usually seems to cast this after the second Shadow Bolt.
- Drain Life - Low damage and channelled.
Teron'gor (Affliction)
- Shadow Bolt - Decent damage, probably better to steal Agony if possible.
Teron'gor (Destruction)
- Chaos Bolt - Yes it does a lot of damage, but probably better to steal Immolate and not let your group members die.
- Immolate - Does a lot of damage.
- Incinerate - Does decent damage.
Teron'gor (Demonology)
- Doom - Does a LOT of damage.
Bloodmaul Slag Mines
Bloodmaul Ogre Mage
- Lava Burst - Does decent damage.
Magma Lord
- Fireball - This is the miniboss in the fire gauntlet. Does decent damage.
Calamity
- Scorch - This is the fire elemental summoned by Forgemaster Gog'duh. Fairly high damage for a single nuke.
Bloodmaul Flamespeaker
- Channel Flames - These are the casters before the final boss. You might need to stand in the fire for a tick in order to get the steal. Does a LOT of damage.
Gug'rokk
- Molten Blast - Does a lot of damage and applies the buff to yourself. If you get 3 stacks, a barrage fires off. Make sure you have a dispel so the boss does not get 3 stacks of his own.
Grimrail Depot
Iron Docks
Ahro'ok Dugru
- Blood Bolt - This is the female Orc caster in the Grimrail Enforcers encounter. Does moderate damage.
Shadowmoon Burial Grounds
Shadowmoon Bone-Mender
- Shadow Bolt - Does good damage.
- Shadow Word: Frailty - Instant cast DoT that does a LOT of damage. Probably best chance to get it after interrupting a Shadow Bolt.
Shadowmoon Enslaver
- Rending Voidlash - Does a LOT of damage, but requires you to channel it.
Shadowmoon Exhumer
- Void Bolt - Does good damage.
Monstrous Corpse Spider
- Death Venom - Does a LOT of damage, but dangerous to let cast on party members.
Plagued Bat
- Plague Spit - Does moderate damage, probably safer to steal than the spiders' casts.
Skyreach
Adept of the Dawn
- Flash Heal - Not really worth stealing.
High Sage Viryx
- Solar Burst - Does good damage.
The Everbloom (You should glyph Dark Simulacrum here)
Everbloom Tender
- Enraged Growth - Cast this on yourself for a significant damage boost. Can alternatively be channelled on allies (or enemies) and stacks with itself if you have more than one DK. Rogues, for example, get a huge benefit from the attack speed.
- Dancing Thorns - Does decent damage.
Earthshaper Telu
- Bramble Patch - Does a LOT of damage, although it does not show up as one of your damage sources in the combat log.
Putrid Pyromancer
- Dragon's Breath - Usually want to interrupt this if it's aimed at party members, but it's even more effective as an additional crowd control tool if you AMS or dodge it. It ticks for high damage as long as you channel it (don't try turning or it will cancel), but only seems to tick on mobs that are not already disoriented by its effect. For this reason, it does a LOT of damage to bosses (yes, you can and should use it on more than one boss).
Upper Blackrock Spire
Black Iron Dreadweaver
- Shadow Bolt Volley - Reportedly cannot be stolen in Challenge Mode.
- Shadow Bolt - Does a lot of damage, safer to grab this.
Black Iron Summoner
- Fireball - Does a lot of damage. Supposedly their summoning spell can also be stolen, but creates an enemy even when you use it.
- Frost Nova - Does a large amount of AoE damage. Looks to be better than Fireball at 3+ targets.
Black Iron Flame Reaver
- Flame Shock - These are the shamans in the room before the final boss.