Our choice: Lobster
Why: Once upon a time, lobster was a luxury item, something you'd eat on a special occasion at the height of summer. Monkfish, often dubbed the poor man's lobster, was considered a middling alternative. These days, though, the Maine lobster population has exploded, with lobstermen hauling in record numbers. The result: rock-bottom prices. You'll pay about $12 a pound for lobster right now versus $15 a pound for monkfish.
The only drawback: None (for consumers), unless you consider OD'ing on lobster rolls a problem.