I kept every single thing I fished this month and made a spreadsheet because I was bored
https://docs.google.com/spreadsheets/d/1jciFHiN1GayaBE4C1A5kO0BOuSdpcNlCoSMrXEvfSPU/edit?usp=sharing
Probably not even a complete list with the rarer fuckers still in the wild
TL;DR
Rare fish are rarer than Epic fish
Parrot Fish canonically best fish
EDIT: Found out there's a gang perk/talent/whatever that boosts fish rarity, and that my gang has it. So that's my baseline rate.