In my opinion there's no wrong or right answer to your question. The best choice depends on many different factors. What are the most important things you're expecting from your API? Is it real-time capabilities, performance, something else?
However, since most languages / frameworks are capable of making developers happy in almost all cases your choice might depend on your personal preferences. If you're already fluent in one language, you might choose that one or if your project is only for yourself you might choose a language/framework you're currently not proficient in.
One, and in my opinion good, option I haven't found in the answers yet is the Go language. It's a clean language by Google, used by many popular projects and is quite approachable to developers coming from any other language. It's also low-level and it's extensive standard library makes it possible to write lots of programs without needing to rely on any third-party dependencies.