Of course it depends on the development platform. While using Windows, I normally have Visual Studio (with a small extension for it), but on OS X the best editor is definitely TextMate. I wrote two short articles about these, you can take a quick look on them here:
Using Visual Studio as a Ruby On Rails IDE http://www.eclips3media.com/workshop/?p=8
TextMate: The best Ruby On Rails editor for OS X http://www.eclips3media.com/workshop/?p=5
Cheers, Attila