The truthfulness of this information depends on what is the definition of genocide for you. People think of genocide as the wiping out of a large population of a certain caste, race, religion, or ethnicity. The real definition of genocide, however, is slightly different. It means wiping out of such a population by a person or a group “intentionally”.
Now let’s talk about the two examples of genocides which you have mentioned in your question one by one.
What happened to Red Indians in Americas is often read and analyzed under the banner of colonialism. “The indigenous peoples of the Americas experienced massacres, torture, terror, sexual abuse, systematic military occupations, removals of indigenous peoples from their ancestral territories, forced removal of Native American children to military-like boarding schools, allotment, and a policy of termination”, Wikipedia tells us.
(Courtesy: All That's Interesting)
The main reason of the deaths on larger level of the Native Americans, however, was smallpox, which hit the ethnic population. Since this reason of mass death is not intentional, I don’t think we can call this a genocide.
There are some historians, though, who maintain that British militia gave the smallpox exposed blankets to the natives to exterminate them but this theory is debatable.
Now coming to the example of Germany, it is needless to say that the Nazi Holocaust is the best documented and perhaps the scariest genocide in the World history.
Look at the following table to see the number of deaths in the top 10 genocides in history.
I don’t think I need to retell what occurred to not just Jews, but almost all minorities and of not just Germany but the whole Europe under the rule of Adolf Hitler. The concentration camps, the holocaust, and the ideology on which the whole set up was based can still give you goosebumps, and the history is very clear about the “intention” of Nazis to wipe out everyone not belonging to Aryan Race.
So historically, the Nazi Holocaust in Germany is the worst genocide until now.