Class
The taxonomic category that contains organisms least similar to one another is the Class. This is because the Class is a higher taxonomic rank compared to other categories such as Genus, Family, and Species.

Explanation:
- Class: Organisms within the same Class may have different orders, families, and genera, hence they may not be very similar to one another. For example, in the Class Mammalia, there are various orders such as Carnivora, Primates, and Rodentia, each containing different families and genera.
- Genus: Organisms within the same Genus are more closely related compared to those in the same Class. Members of the same Genus share more similarities in terms of physical characteristics and genetic makeup.
- Family: Organisms within the same Family are even more similar to one another compared to those in the same Class or Genus. Families consist of multiple genera that share common characteristics.
- Species: Organisms within the same Species are the most similar to one another as they can interbreed and produce fertile offspring. Members of the same species share the most similarities in terms of physical traits and genetic information.
In summary, the Class contains organisms least similar to one another as it represents a higher taxonomic level where organisms may have more differences in terms of characteristics, behavior, and evolutionary history compared to lower taxonomic categories.