The team from The Coming Storm doesn't count; all heroes have 4-5 missions (including the final ones), so even the craziest skill experiments work there.
Having spells for all occasions is great, but it's often not necessary. If you don't mess with the Storm campaign, the army acts as the missing spells for all occasions.
Zy's answer, Zy:D. The fifth level of magic might actually hinder you. For example, Order against Death; Order has "Mirror Image," "Hypnotize," and "Steal All Spells" at level 5 – only the first of these three might be useful, and even then, not very much. And it takes 5 levels to get from level 4 to level 5 magic (without altars)! You have to agree that's a lot. It's not difficult to gain 5 levels at the beginning, but what if it's levels 20, 21, 22, 23, 24? Then every thousand experience points counts. So, the fifth level isn't always needed, but it probably doesn't apply to my example in the previous post.
In general, IMHO, you really need to look at the circumstances. If your army is small, level up a druid or demonologist; if it's larger, focus on blessings. If the enemy army is large and has no immunities or resistances, then combat magic or curses will be useful (provided the enemy doesn't dispel them or redirect them at you). The same applies to non-magical skills. If you want a powerful hero, you need Warfare; if you want to strengthen your army, level up Tactics; if you don't want to fight mine guards and want to learn about the enemy, take Scouting; if you need more resources and creatures, you need Nobility. Although a lord isn't needed in the army, as Alex_P noted, he shouldn't be your main hero – let him sit in the rear and accumulate money. :D
🏹🎪+⛓️♿=💕