Three spring to mind:
In Cuba ... where there are many, many really good eating fish to be caught: the tail-end fillet from a fresh red snapper (tail-end as the grain in the meat is finer; true, deepwater red snapper because this is, I think, the finest eating of all the snappers if you bleed the fish then ice it straight away) ... just basted in mojo de ajo (garlic butter) and griddled till done, served with a lime wedge and a pinch of salt, rice and fresh salad. Simple, and superb.
On a boat in Pacific Mexico we caught a small yellowfin tuna and the crew asked if we wanted ceviche. Yes please. Tuna fillets were put on ice; shallots, local tomatoes and ripe, soft avocado were cubed, a green chili was chopped, lots of fresh limes were squeezed, and salt, worcester sauce and fresh pepper were added, then the tuna meat was diced and the whole lot mixed together and left to marinate for 30min. That allowed the champagne we'd brought to chill in the ice chest perfectly. Served with proper tortilla chips and ice-cold champagne in the hot, dry sunshine ... magic. After scoffing the ceviche, you drank the juice - it was like drinking pure zingy vitamins and you could feel your body rejuvenating with all the good stuff you were taking in.
Lastly: tail fillet from a 6lb cod off a Channel wreck (the best eating size in my humble); bled, iced then taken to a good local chippy straight off the boat, fried in their batter after asking nicely, and served with thick chips. I think there's a wonderful creaminess about cod this size less than 24hr old, and once again, the meat from the tail fillet has a finer grain.
The best-tasting fish is the one you catch and prepare yourself. You know how fresh it is, what's been done to it and how you want it - and there's nothing like it when it's done simply.
Hungry now ...