Sea Dart and Sergeant York as oversold weapons

What weapon was most oversold and painfully expensive for what it delivered?

Sea Dart is the first answer, with Sergeant York also accepted as a plausible example.

Sea Dart is the immediate choice for the most oversold weapon, with Sergeant York accepted as another strong candidate. Nothing here establishes that either system was useless or compares their combat records in detail. The point is the gap between expectations, expense, and delivered capability.