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In Heroes of Might and Magic I, I like all the units; they are designed in a fairy-tale atmosphere, and each unit is memorable (especially since there aren't that many of them).
In Heroes of Might and Magic II, I like the liches, vampires, dragons, and human units (except for the peasants).
In Heroes of Might and Magic III, the Barbarians are less appealing – it's plagiarism upon plagiarism, and they're in gray colors. The Castle and Inferno factions are amazing; there are no words to describe them. The gothic style in Heroes is something special. A fairy-tale atmosphere is essential for the game because it's one of the reasons why everyone loves Heroes. But here, it feels like NWC hired a priest as the head of the art department. And the same goes for the Swamp faction.
In Heroes of Might and Magic IV, the dragons are especially good, particularly the magic dragon (at least here, it looks like a dragon and not like a barrel with butterfly wings, as in Heroes III). Who said that the scariest monsters are humans? In Heroes IV, the scariest monsters are the heroes, but they are also the most interesting. Especially the sorceresses in wigs.
In Heroes of Might and Magic V, the Orcs, Humans, and Inferno factions look solid. The Vampires and Dragons are also great.