Audit trail files capture chronological records of system activities, crucial for security, compliance, and troubleshooting. The "best" format balances integrity, readability, and portability. Text-based formats like CSV (Comma-Separated Values) and JSON (JavaScript Object Notation) are popular as they are human-readable using basic tools and allow easy parsing for analysis. XML offers strong structure and validation but can be verbose. For high-volume systems, binary formats like proprietary database write-ahead logs (WAL) offer compactness and speed but lack easy human readability without specialized tools.

CSV is widely used for exporting audit logs from applications and network devices due to its simplicity and compatibility with spreadsheet software. JSON is a common format for application audit logs, especially in web services and cloud platforms (like AWS CloudTrail logs), as it easily structures complex event data. Database management systems often employ specialized binary WAL formats internally for transaction integrity and performance. Industries prioritizing compliance (finance, healthcare) favor formats ensuring immutability.

Key advantages include CSV's universal readability and JSON's flexibility; limitations involve CSV's lack of inherent structure and JSON/XML's verbosity impacting storage. Security is paramount: formats must support mechanisms like cryptographic hashing to prevent tampering. Future trends involve standardizing structured log formats like CEF (Common Event Format) or using managed cloud logging services that abstract format concerns, enhancing scalability and centralized analysis while ensuring robust data protection.