According to Zaroff, the most dangerous game is man. The title provides a play on words. Some, without knowledge of the story, may think that the story "The Most Dangerous Game" is an actual game played. While this is true, the most dangerous game also depicts the man Zaroff is hunting.
Game, when defined, is an activity engaged in either amusement or as a diversion, or it is an animal being hunted or taken as the result of a hunt.
"I hunt more dangerous game."- Zaroff
Zaroff goes on to describe his game (the animal as well as the activity).
"I had to invent a new animal to hunt...And the answer was, of course, "It must have courage, cunning, and, above all, it must be able to reason."
In the end, Rainsford is the one to let readers in on Zaroff's new animal:
"But they are men," said Rainsford hotly.
Basically, Zaroff defines the most dangerous game as a man. The reason he justifies this is that only man is able of reason. For that reason, man is the most dangerous game.
To General Zaroff, the most dangerous game is men. The reason for this is because they can reason.