Chain Lightning - This spell actually comes with a long cast time (2 seconds) so I like to pair it with abilities like CoC and Blizzard.
Virulent Walking Bomb - More fun than useful in most situations, it still brings heavy Physical damage (but not reduced by armor).
It is worth mentioning that this is the easiest way to get your Heavy Hitter achievement.
Mana Clash - While technically AOE, this spell feels more like straight dps vs Casters.
While Inferno is technically a better choice, I often don't have the points to go that far down the fire tree.
In the same mind set of CoC into Shock: Blizzard into Tempest.
Winter's Grasp - Though at later levels it has a low freeze chance (100-enemy level*5), and it doesn't freeze enough to shatter, it is very cheap and is useful for a large portion of the game.
It is cheap, effective, aoe stun and I cannot imagine skipping it.
Mind Blast - Every Mage I make has this ability.
Since FB is low damage and CL is expensive, often times my combo is just CoC into Shock.
Cone of Cold - Great as a CC and for Shattering people, but I find the real power of Cone of Cold is lining up my enemies for Shock, Flame Blast, Chain Lightning (assuming you have the mana) This is just powerful dps and you don't have to worry about enemies moving.
It is invaluable for removing powerful casters.
Crushing Prison - Both good damage and powerful cc, Crushing Prison will remove an enemy from the fight for a long time and even shatter frozen targets.
Good and fun are a bit subjective, but I can tell you what is powerful.