Depending on the requirements of your REST API project:
If it's a small project that doesn't need sessions/cookies, database integrations ...etc, then the standard library is sufficient.
One step up from the standard library would be an unopinionated framework like gin
If it's a big project with time restraints/deadlines or requires features like database integrations, cookie/session management ...etc, then go with buffalo.
Depending on the requirements of your REST API project:
yeah, since the project I am going to apply Go is quite simple. I actually have been working MUX.