The greatest diversity of animal and plant species is found in :

The greatest diversity of animal and plant species is found in tropical moist forests, particularly tropical rainforests. Biodiversity is not distributed evenly across the globe; it is consistently richer in the tropics and decreases as one approaches the polar regions [3]. Tropical rainforests are estimated to house approximately 50% of all living animal and plant species on the planet, despite covering less than 6% of the Earth's surface. These ecosystems are characterized by high annual rainfall and warm temperatures year-round, which support a vast array of life forms, including two-thirds of all flowering plants. While other biomes like temperate forests and grasslands support significant wildlife, they possess lower overall species richness compared to the equatorial tropical forests. Consequently, tropical moist forests are recognized as the most biodiverse terrestrial ecosystems on Earth [2].

Sources

1. [3] https://www.khanacademy.org/science/biology/ecology/biogeography/a/tropical-rainforest-biomes
2. [2] https://www.nps.gov/teachers/classrooms/wildlife-of-the-tropical-rainforests.htm