Which AWS service is BEST suited for building a recommendation engine?

Amazon Neptune is the best choice for building a recommendation engine because it is a fully managed graph database service optimized for storing and querying highly interconnected data. Graph databases like Neptune are particularly effective for applications such as recommendation engines where relationships between items and users are crucial.

In a recommendation engine, understanding the connections between different entities (like users and their preferences or products they have viewed) is vital. Neptune supports query languages like Apache TinkerPop Gremlin and SPARQL, which allow developers to perform complex queries to uncover relationships and patterns in the data. This capability is essential for generating personalized recommendations based on user behaviors and interactions.

While other options like Amazon RDS provide relational database functionality, they are not specifically designed for the graph-like data that recommendation systems often require. Amazon S3 is a storage service, making it useful for data storage but not for building the logic of a recommendation engine. Amazon EC2, which offers compute capacity in the cloud, can be used to run applications but does not inherently provide the database capabilities that are optimal for managing and querying graph relationships.

Thus, Amazon Neptune stands out as the most suitable service for this specific use case focused on recommendations.