There are a lot of healing cards out there, and most aren't that great. Some, like Spirit of Humanity or Ancient Stone are at the top of the list because they can heal without a direct action cost to the player. Of the cards that convert clicks into raw healing, IME Earthly Serenity is probably the best of them.
Drop it for 2 resources for one action, blow up all 4 charges in a test for another, and ... that's it, you're done. It's a strong way to pull an ally out of the danger zone, fast. Compare to First Aid - play for 2, click 3 times, and ultimately heal one less. That said, it being a test complicates things: luck of the draw can have you fail to empty it in one go, although Mystics and Survivors easily have the to make that a rarity. And if you're running Hard or Expert tokens, drawing any symbol token can have consequences that easily outweigh 4 - or even 6 - healing in one action.
You'll probably pick it up mid-campaign when people are traumatized, but it gets good mileage - and the art is gorgeous.