I've already given a head nod to Tempest, but I wanted to make sure I gave a shoutout to the spells that people really love. When new spells are released, it seems to be these types that create the most thunder, and when your class's shiny new damage spell isn't one of these types, you notice. :) They are that loved, and that awesome.
Specifically these bad boys come to mind:
Rain of Fire
*I've probably missed a couple . . . I'm a bit rusty here, and I didn't add anything like the snowball pet's aoe spell, which is something to bring up another day :)*
They're all awesome!