Destruction warlock with big chaos bolts.
Frost mage with glacial spike shatter.
Ele shaman with an elemental blast.
Balance druid with a full moon.
Devoker with a fully charged fire breath.
One of those is probably gunna be your biggest single magical hit you can do. I dunno which hits hardest, or even if some of these talents like glacial spike or full moon are even meta right now or not. There are just the biggest single hit magic abilities I could think of.