The most USELESS monster.

even if this fairy doesn't block anyone from the archers and gets killed along the way, and your forces are too uneven, and there's, for example, a magic gate in the city, then she's still very useful – you'll lose fewer warriors. And even if the forces are equal, the opponent will lose a turn by killing this fairy. As for the mages, I also refute that statement, because with the Ring of Great Negation, the spell affects everyone.

Added 5 minutes later
And the most useless monster, in my opinion, is the one that doesn't fight for you or doesn't fight at all.