Variegated Lardfish

This is the summoner's fish. Summoning Potions have no other reliable source, and every one you cook into a Seafood Dinner is a potion you will not have for the next boss. The food buff is available off plenty of other ingredients, so there is no argument for spending the fish on dinner.

They live in Underground Jungle water, which means either fishing the natural pools down there or hauling buckets into a pit you dug inside the biome. Standing still in the jungle before Hardmode is a bad idea, so wall off a small room around your pool and fish from inside it rather than casting in the open.