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IMHO, it's the Ancient Behemoth. In a 1-on-1 fight, it loses only to the Titan at level 7, and in a 10-on-10 fight, it can defeat Titans and Phoenixes. But the Behemoth costs 2000 gold and 1 crystal, while the Titan costs 5000 gold and 2 dragonstones. Also, the Titan can defeat more than just two level 7 units. 36 Ancient Behemoths can kill 10 Azure Dragons in 99% of cases. No other level 7 unit can do that.