Well, there is not much choice, if we're talking browser based. I admit to have suffered from a mild jQuery addiction, but I'm over it. I'm intentionally not talking of frontend-backend here, for the obvious reason that a lot of the logic except data persistence has been outsourced to the browser with the modern ways the language has taken in e.g. simulating object oriented class structures.
But I've also built android mobile application clients, which means Java or Unity3D "clients", which means C#.