I voted for Immolation, Rain of Fire, and Deflagration.
Immolation: Great quanta accelerator; the only other card in the game that can compare to is is nova.
Rain of Fire: One of very few mass creature removal cards. (the others are pandemonium and thunderstorm)
Deflagration: Who doesn't love permanent removal? This one's so good, I've actually made decks that were all-mono, except for mark of fire and a couple of these.