Without looking at the exact numbers I would guess the angler fish. Legendaries are always a good choice - if you want to see the exact numbers: http://stardewvalleywiki.com/Stardew_Valley_Fair Click that and expand the Fish section, it'll tell you the values for everything. Good luck!
An Ice Pip or Lava Eel is probably the best. Worth 6 points like the legendary fish, easier to catch than most of those same legendary fish, can be found year round and are replaceable if lost on the display. (That is, if you forget to clean it out.) Plus, they are worth 6 points at any quality, so gold is not required. For reference Ice Pip is level 60 of the mines and Lava Eel is level 100. The Ice Pip is the easier of the two to catch, but fair warning both can be rare. Angler fish is also worthy, it is an easier catch (especially compared to other legendary fish), happens to spawn in the fall. But, I personally don't like to risk it because its a one time catch. Even though I am super careful to never forget the display, I still feel safer with the first two I named. (Or the Legend, but that is an entirely different story.)
Luthien gave you the link with all the scores, you don't need to reach max score with everything, just a few things, normally I use a crab for the display lol Also there's a calculator here.
