Question

According to General Zaroff why is Rainsford the ideal prey

Answer 1

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.
~
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.