The correct service that offers detailed analytics for expenditure and usage across multiple accounts, and allows for the application of custom cost allocation tags, is AWS Cost Explorer. This tool is specifically designed to help users analyze their spending and usage patterns, make informed decisions about where costs are incurred, and look for ways to reduce unnecessary expenses. AWS Budgets is used for setting up custom budget alerts, not for detailed analytics. The AWS Billing Dashboard gives a high-level overview of costs but lacks the detailed analytics and customizable tagging capabilities of AWS Cost Explorer, and the AWS Pricing Calculator is for pre-usage estimates, not for post-usage analysis and management.