it depends on what you do, this is how i do it, i'm not really a battlemage, i use some light armor along with a robe but this is what i use

glass helmet - destruction fortification and illusion fortification
master robe of destruction - destruction fortification and magic regen
necklace - destruction fortification and alteration fortification
ring - destruction fortification and conjuring fortification
glass gauntless - light armor fortifcation and magic
glass boots - muffle and stamina
thats my setup and i use a few followers and they each have enchanted gear suited just for their playstyle for when i decide to use em, i also use the lordstone which gives a 50 bonus to armor and 25% bonus to magic resistance along with my breton racial traits which give me 25% more magic resistance and dragonskin which absorbs magic and its useful once in a while, especially at lower levels.