I’m not sure that the resources on this list are really the “Best,” but they are the ones that I’ve shared in the past related to using primary sources in lessons. Feel free to share additional suggestions.
Digital Public Library of America (DPLA) Primary Source Sets are designed to help students develop critical thinking skills by exploring topics in history, literature, and culture through primary sources. Drawing online materials from libraries, archives, and museums across the United States, the sets use letters, photographs, posters, oral histories, video clips, sheet music, and more. Each set includes a topic overview, ten to fifteen primary sources, links to related resources, and a teaching guide.