Пандора
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The question sounds strange in general. And in what sense "powerful" - if in terms of strength, then definitely not a Fairy, in terms of usefulness - I think the Fairy has helped many, - no answer, and you can also do one trick with her due to the fact that she flies, but in general, I prefer Wolves and Arbalesters.
There is a separate topic on usefulness. You can also participate in it.
I personally like Archers - Halflings and Snipers more. Also Wolves and Fairies. And it's not that I evaluate them by strength, but rather by maneuverability... I am a strategist :)