Species (The variety of Life Forms)
Species do not interbreed. Taxonomy arranges species in a hierarchy.
Ecological Functions (that species perform)
Functions include predator/prey dynamics (see Foodweb)
Genes(The genetic diversity they contain)
Genetic diversity is important within species (avoid inbreeding and bottlenecks)
More value is assigned to "rarity"; rarity is partly defined by the following concepts:
Range (Endemism, meaning "restricted in range"): Black-billed Parrots (endemic to Jamaica) are more valuable (ceteris paribus) than Cattle Egrets, which are spreading throughout the World.
Higher in the Taxonomic hierarchy is better (ceteris paribus). So Jamaican Tody species, which belongs to a Genus that is endemic to the Caribbean are "better" than Turkey Vulture species which is endemic to the Caribbean but belongs to a genus found elsewhere.
More-Ancient lineage is arguably better.
Unique or special adaptations of a particular species have value eg Crab Sesarma windsorii??.
Uniqueness of a functional group has value. Eg Top Predators, Keystone species