Claude Sonnet is the strongest choice for writing a cover letter because it excels at nuanced, professional prose and can tailor tone and content closely to a specific job posting when you paste it in. It follows instructions precisely, making it easy to highlight relevant experience and match the employer's language. ChatGPT is a solid alternative with a similarly strong writing ability and a familiar interface many users already have access to. Gemini Flash works well if you want a free option with no signup friction. Jasper is worth considering if you plan to write many job application documents and want templates built specifically for professional writing.