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1. Life. Regeneration is king! The ability to resurrect creatures after battle is useful.
2. Haven. Hypnosis, phantoms, charm... No contest here. Could easily take first place, but regeneration is really powerful!
3. Sylvan. Summoning is summoning, and minions are always helpful. Especially if they are water elementals ;) Although, there can be even better options if you invest in it. The downside is that Nature uses it really effectively, while other cities are not as...
4. Necropolis. There's plenty to work with here. Overall, it's similar to Nature. It has its own advantages, of course, in exchange for some of Nature's advantages. The main downside is the same. Although necromancy with vampires is cool, I'm not sure if multiplying vampires while playing as Life will be useful (although I haven't tested it!).
5. Dungeon. Offensive spells in this game are boring. No interesting post-effects. And no skills that are useful on the global map...