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Considering all parameters, Assassins are definitely the best. They don't shoot very well, but they poison effectively, look good, and in 3.0, the alternate form completely disappears – which is useful. Peasants are also decent in the early stages of development – you leave them in the city, and gold accumulates. Gremlins are nonsense; in a good battle, their skill is useless because they simply don't have time to act due to tactical considerations, and even if they do, it's already too late – a Golem isn't that much of a powerful unit anyway. Imps are only useful in pairs with an Archdevil.