[thejeff]

Best is not even a judgement call. It varies from nation to nation and unit to unit.

E9 is great with high protection units and mages.
If you've got low prot troops and no sacred mages it's useless.

Water is always good, but the defense bonus is best with high defense troops. If you've got a low defense, you'll get hit even with the +3.

The regen from nature is great on high hp troops, but even the lesser chance of afflictions isn't useful on something like flagellants. They die if they get hit.

Twist fate doesn't really help Neifel giants, but it doubles flagellants lifespan.

etc, etc.

Water and fire are probably the most generally useful. Everybody can benefit from more damage and more attacks. But they may not be the best for a particular nation.