Java
Typically, language is not the determining factor in developing or programming a GUI. More importantly is the application that determines the development platform you are using. At that point, language is not so much of a factor. Depending on the application and development platform, certain languages are better suited though, so once you choose your application, then you pick from the languages that are available. Still though, ease should not be a factor, but servicibility and reliability are way more important than ease of programming. If you can eliminate all other factors, than you can pick the simplest platform available, usually the highest level programming language that has everything you need is a good choice.
That said, some programmers will just go for the simplest platforms every time and try to shoehorn it into the problem they are trying to solve. Java's as popular choice for some apps, WinForms on .NET are also really simple for Windows apps, you can use various languages for that though, C# is pretty easy if you have the tools. Really depends on what you want to do for your GUI though, so there's not really one answer. You have to answer questions like, Is it a Web app, Phone app, Windows app, Apple App, or what? That really determines your programming language.
