For me, the best visual Swing editor is JFormDesigner, which you can run standalone or as a plugin for IntelliJ IDEA and Eclipse.
It generates proper (actually readable) source code, it's very ergonomic and intuitive and, above all, very extensible. That last point is really important, because if you want to build a decent Swing application, you'll have to extend the base components or use some third-party libraries and it must be easy to integrate those in the visual editor.
It's not free, but it's a bargain for the power you get (129 EUR / 159 USD). I've been using it for a few years and love it.
