Depends on 1. your personal prefference. 2. were you want to spend your perks
For Master diff, i recamend destruction (pick your fav flavor of damage and 1 secondary), backed by conjuration (atronach side+twin souls), Quiet Casting( illusion perk, lets you cast all spells silently), Necromage ( Restoration perk, all spells work 25% better vs undead. yes i'm looking at you Mr Death Overlord) Alchemy for general utility (spell damage boosts, invis for no magicka ect) and stealth for both getting out of sticky situation and ambushing the enemy with destruction spells.
Breton is best as a defensive mage (atronach stone, necromancy, alteration perks). Altmer for Offensive. The Highborn ability refills your magicka like no ones business for 60 seconds of godlike blasting.