I didn't look at anybody else's answers, but I think the following would make an interesting cast.
Whitney: Someone like John Cusak would be good. He's got the ability to carry the part without taking over.
Rainsford: I think Brad Pitt could carry of the roll quite convincingly.
Ivan: I'm not quite sure who would be good for this, but I would like to see someone new, rather than a traditional Hollywood roll.
Zaroff: Malcom MacDowell or Anthony Hopkins would be chillingly convincing.
IVAN: While he is not an actor, Vladmir Klitscho, the Russian boxer seems an appropriate physical match for Ivan. All that is needed is physical presence and Dr. Klitscho certainly has this: He is a muscular heavyweight at six feet six.
GENERAL ZAROFF: Nicholas Cage seems capable of playing the role of a sadistic predator who is jaded in the ways of the cosmopolitan world.
RAINSFORD: Leo DiCapprio, a versatile actor who has the capabilities of protraying a character who changes from predator to "beast at bay" back to predator.
WHITNEY: A minor role such as this can be played John Cusak.
Rainsford should be played by someone who can be brash and arrogant as well as ingenious enough to outwit the master hunter. I know he's considered a "pretty boy," but how about George Clooney.
Zaroff needs to be played by someone who has the veneer of a cosmopolitan, world traveler but the fierceness of a cannibalistic hunter. My vote is Kenneth Branaugh.
Ivan is ruthless and cruel but also dispassionate. He has to be able to speak volumes without saying a word. Christopher Walkin would be amazing.
Whitney could be anybody.
How about Sean Connery as Zaroff and Jude Law as Rainsford? Although I do like the previous post's idea of John Malkovich for Zaroff...he always makes such a great evil character on film!
The guy who plays Snape in Harry Potter would be a good Zaroff, too. He's got the look of a potential evil/intellectual/manic killer down pat.
I can tell you that the first film adaptation of "The Most Dangerous Game" from 1932 starred Joel McCrea as Rainsford; Leslie Banks as Zaroff; and Noble Johnson as Ivan. Trevor Howard played a Zaroff-esque character in the Mexican-filmed Run for the Sun, based on the short story; Richard Widmark played the character based on Rainsford. It was also remade in 1945 with a World War II theme that include a Nazi madman instead of a Cossack.
I would probably cast one of my favorite actors, Christopher Walken, as Rainsford (though he would also make an interesting Zaroff). John Malkovich would be my choice for Zaroff, and Ivan would be played by Governor Arnold Schwarzenegger. Whitney, a truly minor role, would be played by Tom Berenger.