I'm thinking the best follower would a tank. I'm also thinking to use lots of shock spells - is there a follower w/ good resistances?

Basically whichever one you make/buy/find good resistance gear for!

Between killing your follower with AOE and healing the enemy instead of your companion (doh!) you may want a ranged companion too and just get some summons to tank.

I found that my enjoyment of the game is 10x greater without a companion for any character. Spend half my time babysitting them, or having them run in front of me at the least opportune moments. I don't even bring them along as pack mules anymore.

Another mage, if you use conjuration and he does, you can summon 2 dremora lords/2 storm atronachs and he can summon an fire/ice atronach, thats a lot of things to sponge damage instead of you. Just an idea, I don't use companions.