Elemental is a pretty weak tree, with relatively low damage coefficients except for Firestorm, no cross class options besides unreliable Brittles and relatively little utility. The best overall tree is probably Primal, with strong cross class combo options with Petrify and Chain Lightning and better damage coefficients than Elemental, except Firestorm vs. Tempest. Tempest covers a much larger area than Firestorm, however, which is arguably more important, lasts longer and often causes the electricity damage mini-stun, making it ideal for controlling entire battlefields of minions. Also, in Nightmare, Primal spells don't have friendly fire, which is more than a little bit important.
Other strong damage spells include the upgraded Horror (often the top single target damage spell, plus a strong stun) and Walking Bomb. Walking Bomb is arguably the most powerful spell in the game, as it does damage equal to the health of the mob you just killed. What this mean is that you can take out groups of lieutenants by essentially killing just one (works extremely well with rogues). Unfortunately, in Nightmare Walking Bomb has friendly fire, making it almost impossible to use. In Normal though it's almost broken - only the relatively small radius and the micromanagement keeps it from being so.
Something to keep in mind is that at the beginning, you really want to be putting all your attribute points into your primary damage stat. Each point of DPS stat gives you 0.5 Base Damage. Later on, when your weapons are 30-40 Base Damage, the stat point doesn't mean as much, but at the beginning, each point you put into your base DPS stat gives you a 5-10% increase in your total damage per point (starter gear is 5 Base Damage). Base Damage affects all forms of damage, both autoattacks, special attacks and spells. Willpower isn't really useful except to hit particular thresholds for activating a series of abilities or to compensate for some sustained abilities.