Not taking anything away from Graves, but if you are looking at this all-time, Jean Ratelle, Andy Bathgate, Rod Gilbert and Harry Howell should be in the conversation. They are all before the existence of an entry draft I believe, so scouting players and signing them as free agents was the way you did it then, but they were signed as free agents nonetheless.
I believe also that Conn Smythe went up to Canada and signed Bill Cook before Mtl could do so and the Rangers paid some other team to be granted his signing rights. If memory serves correct, I believe Bill Cook was the first player the NYR ever signed. I could be wrong on that.