What are the two meanings of the title, "The Most dangerus Game"?

The two meanings are based on the word "Game".  A game can
be something played for enjoyment, or " animals under pursuit or taken in hunting;
especially : wild animals hunted for
sport or food" (see link below). 

Zaroff tells Rainsford
"No animal had a chance with me any more.  That is no boast; it is a mathematical
certainty. the animals had nothing but his legs and his instinct.  Instinct is not match
for reason."(pg 5)  So Zaroff went in search of better game, and he found it.  He says
"it supplies me with the most exciting hunting in the world.  No other hunting compares
with it for an instant.  Every day I hunt, and I never grow bored now, for I have a
quarry with which I can match my wits." (pg 6) That quarry is the only one that can
reason; that quarry is man.  Since man can reason, he can match Zaroff's wits, and
becomes the most dangerous game to hunt.

The second meaning
is the game that is played.  Zaroff has decided to hunt Rainsford.  He tells him "You'll
find this game worth playing".(pg8) Although Zaroff does not admit it at first, the game
is to the death.  That makes it the most dangerous game.  Only one man
survives.