I consider the stdlib to be Go's best feature too. As a somewhat contrived example, can you name any language that lets you write a HTTP/2 TLS endpoint that computes the HMAC of a PNG file's pixels without any dependencies? And if something is still missing, it's probably in golang.org/x (which is basically stdlib)!

After that, I probably consider readability at 3am [1], defer statements, explicit error handling and fast compile time to be the most important.

[1] Readability of not just your code: being able to go-to-definition into stdlib and immediately understanding it without having to grok a million unrelated decorators/FactoryFactoryFactory/std::_Vector_iterator<std::_Vector_val<std::_Simple_types<<block>>> is incredible