The one that gets you a proper answer. Tell us the file type, its rough size, your macOS version and what the tool actually said. The first reply then usually solves it.
support@macxtools.comBest for the short question before you buy. Whether a format is supported, whether a tool will touch your original, whether it is even the right one.
Monday to Friday, business hours IST
The same people who wrote it answer it. No support tier, no outsourced first line, so a reply can say what the tool is doing rather than what the manual says. More about the team.
It reads your own file and shows you what is inside before any money changes hands. If the result confuses you, that is exactly what the email address above is for. Many answers are already written up in the MacXTools guides.