What type of application would enhance user experience in Salesforce while integrating an external data system?

A Canvas application is designed to enhance user experience in Salesforce by seamlessly integrating external data systems within the Salesforce interface. This integration allows users to access and interact with third-party applications directly alongside Salesforce data, creating a more cohesive and streamlined user experience.

Canvas applications enable rich user interfaces and interactivity, making it easier for users to engage with external systems without needing to switch between different platforms. This approach leverages the power of Salesforce's existing UI framework while allowing external data and functionalities to be presented in a familiar environment.

In contrast, a Custom API primarily serves as a backend integration method, providing endpoints for data communication without directly improving user experience. REST services also act similarly, facilitating data transfer between systems but not enhancing interaction directly within the Salesforce user interface. A Batch job focuses on processing large volumes of data in the background, which may not be directly conducive to improving the user experience when interacting with external systems.

Therefore, a Canvas application stands out as the ideal choice for enhancing user experience in this scenario.