Fresh produce and ingredients is always important, but never more than with fish. So how can you tell that you're getting the best fish?
Smell : If the fish smells fishy, then it's bad. It's stale and that will not taste good. Good fresh fish will have a salty, seaweed aroma. Often times the fishiness associated with a fishmonger's shop is the waste fish (skin/bones/etc) lurking somewhere behind the scenes.
Eyes : Should be clear, bright/sparkly and full (not depressed and deflated).
Skin : Shiny with clear slime (not cloudy).
Gills : Bright and red showing that there's plenty of oxygen in the fish, meaning it's super fresh.
Touch : Fresh and springy, it should not leave an indentation when you push on it with a finger.