The Great Gatsby is the greatest book ever written.
I don't think many would make that claim. It's a wonderful novel, very short and simple, with memorable characters and wonderful prose. What it does get called is the realization of the Great American Novel that every American writer wants to pen. He broke down the American Dream in such a perfect way, tragic and hopeful at once, and he wrote it at the perfect time to do it, at the dawn of the America's first wave of insane wealth and just before it's steepest fall.
Tons of other books I think would be considered better: Tropic of Cancer, The Heart is a Lonely Hunter, Huckleberry Finn, Lolita, To Kill a Mockingbird, Cat's Cradle, Light in August are a few American tomes that come to mind that I'd put above Gatsby.