It depends on who you're playing against. For example, Nature versus Chaos. If you build Magic Dragons, and Chaos has Black Dragons, then the Black Dragons will be much more useful because of their magic resistance. But I think the most useful units are:
1. Djinn, because of the "Create Illusion" spell.
2. Black Dragon, because of its magic resistance and strength.
3. Magic Dragon, because of its spells.
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