I haven’t really found a definitive answer to this…
Back at the end of TBC, it was fire mages. Then came Wrath and we started seeing the rise of the Frost-Firebolt (Crit) spec. Then as gearing continued it was pure Arcane. By the end of Wrath, Arcane was the accepted raid spec. Fire or Frost-Fire were considered ok, but sub-optimal and frost was… well it was what it always was. (Something Blizzard wanted us to use in raids, but was generally considered a DPS loss by most of the community.)
So, what is it now?
Arcane seems to top the DPS meters, but not the Damage meters. Fire seems to be good for multi-mob bosses, but does it stack up head-to-head vs Arcane on a pure 1-target fight???