"The Most Dangerous Game" is a "game" between Zaroff and Rainsford, the hunter and the hunted. Zaroff desires to hunt an "animal" capable of giving him a thoughtful and challengeing bout of hunting, while Rainsford is left to fill the bill of that "animal." In the end, however, there is a reversal of roles that sees Zaroff in the role of the hunted and Rainsford in the coveted role of hunter. This is absolutely the most dangerous game of all because the outcome is death for the loser and life for the winner; therefore there could be no more dangerous game than this.
Just a note to the response above, Zaroff's most dangerous game too could refer to his hunting of Rainsford, for Rainsford is his equal in every way as a hunter and survivor. Zaroff's recent victims have ceased giving him any sense of competition or thrill of the hunt. So Rainsford is really going to be his most dangerous game.
The most dangerous game was created by General Zaroff. He traps or lures humans to his island so rightly named "Ship Trap Island" by sailors. Once they have been captured they are given a knife and Zaroff explains the "rules" to his "game". The human is given three days to elude capture an death while Zaroff hunts them. If they escape capture and death at the close of three days they are set free, if not- well they die.
The Most Dangerous game also has another meaning and it is that humans are the most dangerous game to hunt. Zaroff was a connoisseur of hunting and had become bored with hunting animals feeling that their cunning could not match his own. In response to his own boredom he thought he would like to hunt humans because they can reason and think and are unpredictable, making them a dangerous game to hunt.
Simply put, humans are the most dangerous game. They have the ability to reason which makes them more dangerous than any other animal.