Shock, easily.
Because it's instant cast and you can hit dragons midair, disingrate perk also works on dragons so no more annoying dragon bones flying around. It also doesn't break sneak. Also only Storm Atronachs have resistance against shock.
Also, using Storm Call shout and then using Lightning Storm is very um, stylish! And I feel like an Sith Lord when using lightning.
Well, you may see this as a bug or something but you really should get %100 cost reduction after level 40 because magic damage doesn't scale and it's kinda sad that you use all your mana, blast your opponent and see that he has 8/10 HP.
I did a pure shock mage build and this is the result;
http://www.youtube.com/watch?v=JuD9phfbKGI
Cost Reduction + Dual Casting + Impact + Shock : FIRINMAHLAZOR
I use them all equally, but casting Fire Storm (+Intense Flames perk) in a town is just hilarious. Combined with Become Ethernal and Mass Paralysis, it's also useful in combat. Other than that, I just love the Whirlwind Cloak from Dragonborn.
I kinda wondered why they didn't add a Wind-based Master spell to the Dragonborn DLC, I mean that would be freaking amazing.