[Alneyan]

I voted Astral, with Death and Air being very close behind. My reasoning for Astral is simply its versatility: Astral offers a very wide array of spells, ranging from Gateway/Teleport to amazing buggs (Will of the Fates). Astral also offers Antimagic and the Amulet of Antimagic, virtual lifesavers when fighting an Astral nation... and Astral also gives the Astral Focus, to go the other way around.

Add to that the Rings of Sorcery and Wizardry to get your other paths up and running (the Ring of Sorcery is especially nice, and not *too* expensive still), and it has to be Astral for me.