Which word best describes having great diversity or variety?

The word "multifarious" is the best choice to describe having great diversity or variety. It specifically refers to something that is made up of many different parts, elements, or types, underscoring the concept of abundance and variety. This term is often used in contexts where the richness and complexity of various components are highlighted, making it ideal for conveying the idea of great diversity.

In contrast, "uniform" refers to being the same or consistent throughout, lacking variety. "Singular" implies uniqueness or being one of a kind, which does not connote diversity. Meanwhile, "monotonous" describes a lack of variety and interest, often because something is repetitive or uniform in nature. Each of these other words emphasizes consistency or a lack of diversity, making "multifarious" the most appropriate choice to illustrate the concept of great diversity.