Well, the very fact that there can be a question like this (and various different answers) shows how good we are in this sport as human beings.
The first genocide was probably the Neanderthals by the Cro Magnions, and we never stopped: the Picts were exterminated by the Celts, the Native Americans by European settlers, the Armenians by the Turks, the list goes on…the unsuccessful attempt by Hitler is obviously the “big star” of genocides, but part of its glamour comes from the very fact that there were survivors to tell their story, which by definition disqualifies it as “genocide” (ie the complete anihilation of a nation). It was more the brutality of the Holocaust, rather than the numbers, that intrigued us, as well as the fact that, being fairly recent, it was fairly well documented. But there are no survivors of the conquest of the New World to share their own horror stories.
The various attacks on the Chinese (the most recent by the Japanese) have had far more victims in terms of numbers, but due to the already huge population of China, went largely unnoticed. And let’s face it, even though nobody will ever admit it, there is different value placed on human life, depending on the nationality of the victim. Americans care a lot more about the death of an American soldier than 200 Iraqis, and a lot more about 1m Jews than 10m Chinese (who nobody would even miss). As for Africans, they are valued around 300,000:1 Americans in the “human life value” stockmarket. Nobody even remembers the thousands of slaves that died in the ships before even getting across the Atlantic, or cares about the genocide in Sierra Leone just a few years ago.