GitHub Copilot is the strongest fit for reviewing code changes before a merge because it integrates directly into pull request workflows on GitHub, can surface potential bugs, suggest improvements, and explain diffs in context without leaving the platform. It understands code semantics across many languages and is purpose-built for developer workflows at a low monthly cost. Claude Sonnet works well as an alternative when you want to paste a diff and get a thorough, conversational explanation of logic or security concerns. Deepseek Coder is worth considering if you want a free option tuned specifically for code analysis.