rarest
adj.adj. describes something that is the most difficult to find because very few of them exist. It is the superlative form of 'rare'.
adj. the superlative form of 'rare'; describes the least common or most infrequent item within a set. Often used to describe physical objects, biological species, or occurrences with the lowest statistical probability.
This is the rarest diamond in the world.
Collectors spent decades searching for the rarest stamp in the series to complete their set.
The conservation team focused their efforts on the rarest subspecies of orchid, which was found in only one remote valley in the Andes.
Superlative form; typically preceded by the definite article 'the'.
the most rarestthe rarestSince 'rare' is a one-syllable adjective, it takes the '-est' ending; using 'most' as well is a double superlative error.