Zaroff, as a master hunter, had clearly become bored with the lack of challenge the natural world presented him with. Ever eager to seek new opponents, he began to consider what would present him with a real challenge. As man has the capacity to reason, he decided that "the most dangerous game" to hunt was man. Thus he created his Ship-Trap Island and meets Rainsford.
General Zaroff's rationale is that the human mind relies on reason and logic as opposed to the animal mind which simply relies simply on instinct. Therefore, this ability to think through a situation is what makes man the most dangerous game.
According to General Zaroff, the most dangerous game is man.
Zaroff states in the story that hunting had "ceased to be what you call "a sporting proposition" and he needed a quarry that could "match his wits". As this is said Rainsford begins to understand that the animal that Zaroff has invented is the human. He figures this out because the human is the only animal that could have courage, be cunning, and above all, must be able to reason. Therefore the most dangerous game to Zaroff is man because man can attempt to fight back or elude his hunter.
Zaroff thinks man is the most dangerous game.