The 3 rarest fish in (Animal Crossing) are the Arowana/Golden Dragon Fish, Arapamia and the rarest of them all the Cruceathan.

Name-Arowana
Time of day-mid day
Season-unknown
value-10.000 bells
Shadow-Unrecognisable
Rarity 9.8
Weather-unknown

Name-Arapamia
Time of day-after 8pm
Season-Augest
Value-10.000
Shadow-Huge(Bigger than U have ever seen can clog up stream when sideways)
Rarity 9.9
Weather-clear days NO rain

Name-Cruciathan
Time of day-9am to around 6/7 pm
Season-All year
Value-15.000
Shadow-same size as Arapamia/Huge
Rarity 10
Weather-Rainy days !ONLY!