It really depends on what you want to fish up. Especially when it is up to rare catches.
Deep water fish: I prefer Fel seas for that. I collect the scales from sea serpents for a lib talisman.
Shore fishes: Switching between Ter-Mur, Illshenar and Malas. Depending on the rare fishes I hope for.
Skill gain (80+): Trap fishing in the lost lands. With some luck a rare crab might show. (With setting multiple traps you can bypass the timer for a fishing pole and get more skill checks over time.)