Q: elder scrolls online best mage build?

Since it’s not a specific question (PvP, leveling, dungeons, trials, …), I’l go with a non-specific answer.

Best mage build is Fire (Destruction) Staff.

You will not get higher damage with any other build.

Abilities you need depend on the encounter and content. You will want Magelight / Inner Light (from Mages Guild skill line). Then look at Destruction Staff skill line, and put Destructive Touch, Wall of Elements, Force Shock, and Impulse to your bar.

Yes there are many other spells you can use, and it all depends on the situation.