My early money is on Shadow Sculptor. That card has been insane for us so far.
It replaces the draw it took to acquire it from the deck, and the coin it costs, but it uses up one of the 5 slots in the city. That can be good or bad, depending on whether you want that battle to be resolved that turn. It's basically a free +1 strength with some control over battle timing. Yeah, that's a pretty nice card. Not amazing, but nice.