I need some advice to choose a language for back-end development. Right now, my REST APIs were created by using Flask/Django, and I'd like to create a more reliable and more efficient API with static typing. On the one hand, Go is young, very light, and syntax like Python's, but C# has a large number of libs and more built-in methods. Which is the best solution today?
It depends.
From times to times I asked or was asked that same question. Technology aside, it's important to consider the skills and expertise that the dev team has. Whether you use language A,B or C or framework X,Y and Z, if your team has a strong background and experience with something make it count too.
I would recommend Go simply because as you mentioned, it's super light. No need to bring in the whole .NET suite to get a simple REST API up and running. Even if your API is a bit complex, Go should be able to handle it.