The current "goto"spec is fire/arcane with mastery-gear.
Arcane for singletarget (gruul, ore, kromog, bh), fire for multitarget (H/F, Furnance, Thogar, Darmac, Maidens).
But learning 2 specs can be "difficult".
If u want only one spec fire should be solid enough.
But it all depends on your needs and your raid. U could ofc go full frost if u like it more and ur raid is fine.
If u want myhtic and stand near or at top all the time fire/arcane is the way.
Fire alone is good at singletarget and god at ae.
Arcane is very good at singletarget but really bad at ae.
Frost is medicore at single and ae. (you will be worth your raidspot but u cant carry someone around)