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In terms of stats, Life isn't particularly strong; it's saved by spells and tactics, but that's a different matter. Based on the stats (if I understand Kennu correctly), Barbarians and Necropolis are the strongest. Necropolis has very strong Dragons, Vampires, and poisonous creatures, as well as skeletons, especially against archers, and Cerberuses are quite tough. In general, races where magic supports the army tend to have weaker armies... Chaos also has a strong army, but magic doesn't help the units much (it's not meant to). By the way, this is a big plus of the fourth game, as I've already said, everything depends only on the player's skill and luck with artifacts.
P.S. I really like Haven now; it's the most versatile faction, although the warriors themselves are quite weak (even the Djinns are only strong because of their magic; in close combat, a level 2 unit can defeat them).