I would say, based on some experience with async capabilities in Python, Java, Go, and C#, that C#'s async/TPL implementation is among the more elegant. Its internals may be complex and it is not without some caveats that must be understood, but I find that it introduces the least amount of extraneous keywords and code constructs in the business logic of an application. And unlike some languages such as Java, C#'s async capabilities are well-supported throughout its core libraries. As someone who builds a lot of data processing applications in Java that benefit from both high concurrency and non-blocking IO, I find myself wishing Java's async capabilities were more like C#'s.