id say master shock destruction spell. Looks like beam *.* AWESOME
Then, cloack spells. I realy like them, give u kind of a "mystireously powerful aura" impresion. Have it ready in case close combat becomes unavoidable for my mage. Then it would come mayhem. Huge range, affects Deathlords too (with master of the mind perk ofc)! Combine it with invisibility, and safely run away in case they keep chasing u after u cast the spell. Then u can watch em fight, and finish em off.
Then, id say wall spells (yes i rly like many spells xdd,) Tought of interesting way to torture ppl. This is combo: Paralize -> Wall of fire -> Wall of frost -> Wall of shock. As a result, ull gain some kind of White Flames Wall xdd
When i first did this on thalmor solider, on legendary, his hp was running out rly quick, poor elvish egoist didnt even get a chance to get up.