The best service to recommend in this scenario is Data Catalog. Here’s why:

• Data Catalog is a fully managed and scalable metadata management service that allows you to discover, understand, and manage your data. Analysts can use Data Catalog to store the results of their PII analysis as metadata associated with the BigQuery dataset. They can add tags, descriptions, and other relevant information to the dataset’s metadata, making it easy to retrieve and understand the analysis later. It is specifically designed to hold metadata, which aligns perfectly with the analysts' needs.

Other options are less suitable:

• Cloud Spanner: A globally distributed, scalable database service, not ideal for storing metadata about PII analysis.
• BigQuery: A data warehouse service where storing analysis results is possible but not as efficient as using Data Catalog for metadata management.
• Data Loss Prevention (DLP): Used to discover and protect sensitive data but not designed for storing and retrieving metadata about PII analysis results.