It's considered a compound adjective when used before a noun, but it's not usually hyphenated.
For example:
* One-stop shop: This is hyphenated because it's acting as a single adjective before the noun "shop."
* A one-stop solution: This is also hyphenated for the same reason.
However, when "one stop" is used independently, it doesn't need a hyphen:
* This is a one stop for all your needs.
* We offer a one stop service.
Ultimately, whether to hyphenate "one stop" depends on its usage and context.