I only want to reiterate that Fire/Water is in one of the most high powered combos in the game
Acid spells are VERY good for their cost and level. Acid Rain can be crazy. The Staff of Corrosion competes with Standard of the Damned for favorite quickened spell slinger item. The only reason these spells don't get more air time is because so very few nations can actually use them.
And actually, I nominate Shinuyama as best nation for making an artillery out of Staves of Corrosion. They have natural F/W gem income, so they don't need to bootstrap one, and 1/4 of mages can naturally craft the staves. Lastly, they have one of the best line-up of commanders for acting as spell slinging artillery (ie. muiltiple high precision commanders, some who can buff themselves with quickness and some who have no magic at all which is actually good in this situation because then there is no worry they will try to cast some other spell.)
This is on top of the other great item available through this combo.
I guess my rating would be:
1. F/W
2. E/S
3. A/N or A/E