To determine what Bishop's most effective simile is in this poem, we must first remember what similes are (explicit comparisons using "like" or "as"), and then look at effectiveness.
In a work of literature, you could use different standards to measure effectiveness. You could just vividness, logic, or power in communicating core themes. I would lean towards blending vividness and communicating themes.
Using those standards, I would say "Like medals with their ribbons" is the most effective simile. It is vivid. It fits well with the lines that follow. It conjures an image, and recognizes that the fish has been in a battle for its life, like a soldier. Medals can be shaped like fish scales, round and overlapping, and so it fits with the literal object being described. It also fits with the claim of wisdom made in the lines immediately following. This fish has been around, fought many battles and learned from them.