As with all good stories, classifications are difficult to make. So, the best answer is to say that the story, "The Most Dangerous Game," fits all the following - imaginary, scientific and informative.
If I had to choose only one category to place the book, I would say that it is imaginary. It is a good piece of fiction. No island like this exists, and the characters are made up, even if there are realistic elements.
If we look at the story from another perspective, there are elements of "science," some may say pseudo-science. For example, Zaroff says that he created a new species of animal to hunt. Here the following words:
The general smiled the quiet smile of one who has faced an obstacle and surmounted it with success. "I had to invent a new animal to hunt," he said.
Of course, these words shocked Rainsford, and when Rainsford found out this new animal was man, he was horrified.
Finally, the story is also informative. It was published in 1924. And this point gives us a window into the culture of the 1920s, when safaris were big among people of wealth in America.