Something Primal by luchia should be cracky, because Castiel turns into a freaking dinosaur to protect Sam and Dean from demon possessed zoo animals. And, to be quite honest, it is. But it’s still so much more than that. There’s something about the way the characters are written that -after I finished reading it-, my heart was aching for everyone. (You can also read the notes for the fic here.)
It’s just really amazing and you should read it.