Congratulations! For me, the most difficult fish to catch, were:
1) Octopus. A real STINKER, i tell you! Once, i found it inside a trash can and i got happy. The thing is more annoying, even than legendary fish, at times. I could never catch it, without a large buff to fishing skill (usually, the Seafoam Pudding).
2) Ice Pip. The thing gave me hiccups, while clicking during fishing.
3) Lava Eel. Nasty, hard catch.
4) Super Cucumber. It hops around like a sore loser, made me very angry.
5) Pufferfish. If i could punch its eye, i certainly would. Nice thing the MysteryMerchant sells one regularly and you can find it from other sources.
6) Lingcod - Catfish. I pit those two together, next to each other. They are ugly, dumb, particularly evasive mofos. I hate them.