whats the best skills/race to be a pure mage in morrowind?

Altmer/Atronach with all magic skills plus one more like athletics (I'm assuming you mean to use no weapons at all) would be the classic pure powerhouse. I prefer Breton/Atronach because they're a little easier to play but they end up with a little less magicka as well.

If you're using GCD, however, I'd recommend and have played a Breton/Apprentice because of the magicka regen with that mod.

I'll endorse Breton/Atronach as well. I haven't done pure magic but it wouldn't be too much of a departure from how I normally play. Illusion is the biggest major skill I'd go with, keeps you alive early on with invisibility and allows you to steal things with short bursts of Chameleon without bothersome things like your actual skills as a thief to worry about.

I've had several go's at pure or pure-ish mages in my time with Morrowind. My most satisfying has been an Altmer with the Atronach sign, majors being Destruction, Alteration, Illusion, Conjuration, Unarmored, and minors being Alchemy, Enchant, Mysticism, Long Blade, Athletics. So not quite a pure mage, with the long blade thrown in there, but you could easily sub in restoration for that. Using GCD, with a bunch of CE enchantments that boost my intelligence, my magicka is around 1500. Personally, I prefer the higher magicka amount that you get with the Atronach sign to another sign which allows for magicka regen (with mods), as I like to be able to throw my weight around in battle. Plus, once you've reached a certain point, the materials for making restore magicka potions are relatively easy to obtain (either by strength or finances), so the regen becomes less important.