Astral, Blood, and Death are the only paths with spells going all the way up to level 9. They are also generally the most powerful end-game paths with things like Wish, Tartarian Gate, and Bind Demon Lord.
Air and Fire are tough to break into, so they're probably lowest on the priority list of magical diversity. If you have them, great. If not, their usefulness is limited to mostly forging. You might have one or two units supporting important armies, but you'll never have them en masse.
The other paths are somewhere in between. They all have useful spells, but you never need a mage higher than level 6 to cast anything. So when you account for boosters, it's not too hard to "max out" on them.
I would say if you don't have natural access to Astral, Death or Blood, those should be your first priority. Sometimes blood is not an option. Second for me is Earth and Nature. Earth for forging, and I just like Nature.
Water, Air, and Fire are my lowest priorities, but having Air magic on an awake SC pretender is a big boon.