What implementation method should be used for real-time quote requests in an online portal?

The use of an Apex callout to send quote requests from Salesforce to the legacy system is the most suitable implementation method for real-time interactions. Apex callouts enable synchronous communication between Salesforce and external systems, which is crucial for real-time quote requests. This means that when a request is made, Salesforce can immediately communicate with the legacy system, receive a response, and display it to the user without noticeable delays.

This method allows for seamless integration and ensures that the user receives prompt updates on the quote, facilitating a better user experience. Additionally, Apex callouts provide the ability to handle complex logic and manage the data sent and received during the communication process, enhancing the overall effectiveness of the integration.

Other methods have limitations that make them less ideal for real-time applications. Middleware tools, for example, typically are better suited for batch processes or asynchronous integrations. Triggers with @Future methods would introduce delays since they operate asynchronously, which isn't suitable for real-time requests. Outbound messages can also suffer from delay issues, as they rely on the Salesforce platform’s infrastructure to process and send messages, making them unsuitable for immediate feedback required in real-time scenarios.