Zaroff has grown tired of hunting animals with lower level thinking. He is interested in hunting humans for the greater challenge of matching wits, or intellectual capacity with his prey.
Rainsford is the ideal prey because he is a hunter too, so therefore, his strategies will provide Zaroff with the maximum challenge for a hunter. He is excited at the prospect of hunting someone who is his equal as a hunter.
In The Most Dangerous Game, Zaroff is an extremely competitive hunter. He has hunted wild animals for many years with great success; however, he is tired of the same experience all the time. To that end, Zaroff has decided to create his own hunt: he thinks that hunting humans is an ideal hunting experience. Rainsford is another great hunter, but he thinks Zaroff is completely crazy. Zaroff is trying to explain to Rainsford why he is the ideal prey for him:
"I wanted an ideal animal to hunt," explained the general. "So I said: 'What are the attributes of an ideal quarry?' And the answer was of course: 'It must have courage, cunning, and, above all, it must be able to reason.'"
Zaroff thinks that Rainsford is the ideal prey because of everything else, he can reason. An animal cannot reason, but a human can reason anything. Rainsford is also just as accomplished at hunting as Zaroff, so this will be the ultimate challenge for Zaroff. Zaroff, however, underestimates his prey this time.
He is challenging because he can reason. Animals are not of any challenge to him anymore because they act off instinct.
Zaroff can assume what an animal will do. He can't always predict human nature.