Java's library is on the whole better than C#'s. But if you want an example of a stdlib that really sets the bar then I would point to Go. In particular the io

Java's library is on the whole better than C#'s. But if you want an example of a stdlib that really sets the bar then I would point to Go. In particular the io packages. They are so composable and simple and easy to use that it makes me want to cry at their beauty sometimes. Go has one of the best written stdlib's of any language I've ever used.