In order to be the highest in DPS and maintain your position in the raids Spell damage and Spell Hit rating must be at the top. Heres a requirements list in order to be prefered.
For Gruul's Lair and Zul Aman :
Spell damage: 950+
Spell Hit rating 80+
Critical Chance 25% +
For karazhan :
Spell damage: 750+
Spell Hit rating 40+
Critical Chance 20% +
I usually prefer Deep fire in 10 man for fast damage and high crits. Ofcourse you must have a high crit chance gear, or else choose arcane fire for enormous blasts but with limited use cause of the three minute couldowns they share.Clear cast must be on every setup for free mana spells.
Subject: Spell damage and Speciality. By Souls 