A fillet knife around six to seven inches with moderate flex, in a corrosion-resistant steel, plus a sheath that drains. Nitro-V is an excellent choice for the job because it combines high corrosion resistance with unusual toughness. Rinsing and drying it matters more than which one you buy.
Six to seven inches suits most freshwater and inshore fish. Too short and you cannot make one clean pass along a fillet; too long and the blade is unwieldy on small fish.
Moderate flex lets the blade follow the ribs. Very flexible blades are specialist tools for large soft-fleshed fish and are frustrating on anything firm.
Corrosion resistance first. Nitro-V is a standout here: its nitrogen addition gives excellent pitting resistance and its toughness is unusually high, which matters in a thin flexible blade.
Whatever you buy, rinse in fresh water and dry it after every trip. See cleaning saltwater residue.
A sheath that holds water against the blade will ruin a good knife. Look for one that drains and never store the knife in it wet.
Add a small sharpener. Fillet blades are thin and come back quickly. See our fishing category.