BestDiff is a browser-based redlining tool designed for legal professionals who require fast, private, and AI-compatible text comparison. It generates lawyer-grade redlines and structured change sets entirely on the client side, with no data uploaded or stored externally; all processing occurs locally in the browser, including offline. The platform produces clean, editable outputs suitable for insertion into emails or documents, and offers AI-native results using machine-readable tags to support seamless change interpretation by LLMs. Features such as multiple diff modes, moved-text detection, selective comparison, and jump-to-next-change navigation enhance the efficiency of the review process. With a zero-upload architecture and strict client-side privacy, BestDiff provides a highly secure and efficient approach to analysing document revisions in modern legal workflows.