I find strategic spells like enchanted hammers, inspiration and propaganda extremely useful. I rarely bother with the other city spells because I've never had a city with enough essence to use more than these 3! I've used tornado to scatter armies getting set to attack one of my cities and pillar of fire to soften up an opponent so the city militia could cope. Regeneration is useful.
For tactical spells haste, slow, heal and burning hands are very useful, flame dart is my principal ranged weapon and can be devastating if you have a chance to pick evoker when leveling up. Sunder will make short work of elementals - including the epic elemental lords, it's probably overpowered. Gift of iron is great if that book is available to you. Most of the others were underwhelming the last time I tried them (back in beta 3) - maybe it's time to give them another chance but the tools I'm using work well, I haven't felt the need.
My current style of play is non-aggressive. I enjoy exploring and questing while expanding quickly enough to stay ahead of the AI so they don't declare war on me. Those who enjoy extermination likely use a completely different set of spells.
