The oarfish, man! I swear I find the stupid thing every time I hunt for a specific fish. Tuna? No, oarfish. Blue marlin? Oarfish. An extremely common and easy-to-find fish that I need to catch to complete a Nook Miles + goal? Oarfish. At least they sell for a lot.
Another thing I learned since starting my new island on the Southern Hemisphere and playing this game in winter for the first time is that ranchu goldfish are MUCH easier to catch in winter because of the limited number of fish that appear in ponds. I always wanted a ranchu goldfish model but was petrified of catching 3 of them since before, with my Northern Hemisphere islands, I spent DAYS trying to get just one of them to donate to the Museum. In winter, I got all 3 of them, as well as the one to donate. Needless to say my mind was blown.