Personally, I go with lighting first and with fire when I have free perk points. Cold sucks - Dragur, nords and many more are resistant to it, and frost spells are too indiscreet. Lightning - isn't the cheapest, but since you won't miss with it, still cost effective. Also, you can shoot airborne dragons with great precision. Almost noone is resistant to it, making it universal. Chain lightning is very handy AOE spell, and Lighting storm is the most useful master. Disintegrate perk effectively takes 15% off enemy health, making it the most and only useful advanced elemental perk. Fire - the cheapest and more damaging element. Handy of foes, most notably dragur and vampires, are weak to fire.