Mutant Carp (I think), because it's the easiest to catch, available in all seasons once the Sewers are open and...
you need to start with the ugliest fish and work your way back. Displayed on a Farm table baking in the noonday sun on the way to the Bus Stop, you can show off your Fishing prowess to all within noseshot. Keeps all but the most adventurous or anosmic villagers well away, and those that power through will run screaming once they catch a glimpse of its lifeless, lolling decomposing eyes.
And of course that winning smile that only a dentist could love.
From there even the Glacierfish is a pale imitator while the Angler, Crimson and Legend are merely delightfully chubby Fish that every child goes googly-eyed over at an aquarium.