What is the MOST accurate Social-Darwinist defense for U.S. imperialism?

It depends on how the Word "accurate" is defined.

If "accurate" refers to a Social Darwinist defense of US Imperialism that was the most consistent with reality and empirically correct, there is no such animal. Social Darwinism is a thoroughly inaccurate description of reality and its defense of imperialism were based on this. It would be like asking, "What aspects of map-making were accurately predicted by Flat-Earth Theory?"

If "accurate" refers to whether an argument is properly attributed to Social Darwinists in defense of US Imperialism, the most accurate such argument would be that according to the Social Darwinist, Imperialism was the natural expansion and success of a well-evolved culture and this expansion would help remove or improve underperforming cultures.

Social Darwinism held that those made better or worse by industrialization were based on the natural predisposition of the people effected. For those who were made better off, the success of those people demonstrated that they had a well-evolved culture and this expansion would help remove or improve underperforming cultures, which were those "chosen" by the fact that they were performing poorly.