Describe the input you have, the output you need, and the constraints that matter. “I need to turn a CSV with quoted commas into JSON while preserving leading zeros” invites useful comparisons; “what is the best converter?” does not.
Say whether the data is sensitive, whether it must stay on your device, how often you will repeat the task, and which tools you have tried. Include a tiny synthetic sample and expected result if format details matter. Check the tools community for an existing answer before opening a new thread.
A recommendation is only useful when it fits your actual workflow and privacy requirements.
Akousa