So, what do you think is the best artifact to have in your inventory? I'm currently thinking Auriel's Shield.

Edit: The second word of the title should be "artifact". (Too late to change it, unfortunately.)

For what game?

The first one in which the artifact in question appears, of course. That 750 point shield combined with spell reflection is just too good to pass up if you're not a caster.

Worth noting that the Oghma Infinium is not the answer, because:
* In Arena, it never actually enters your inventory.
* Even in later games, where you can actually get it in your inventory, it's a consumable with a permanent effect, and therefore doesn't remain in your inventory.