fastest
adj.adj. moving or happening at the highest speed compared to everyone or everything else. It is the superlative form of 'fast'.
adj. the superlative form of 'fast', indicating the maximum speed or shortest time relative to all others in a group.
He is the fastest runner in our school.
The cheetah is the fastest land animal, reaching speeds that allow it to catch almost any prey.
Engineers are constantly competing to design the fastest processor, knowing that even a millisecond of improvement can provide a significant edge in high-frequency trading markets.
As a superlative adjective, it is almost always preceded by the definite article 'the'.
He is the most fastestHe is the fastestShort adjectives like 'fast' use the '-est' ending; adding 'most' is a double superlative error.