That's probably two questions.
1.) What are my Mono GUI RAD options?
If you want a Mono GUI RAD your only choice is GTK# as far as I know.
If you want a GUI RAD for Windows.Forms, it's called Visual Studio (or potentially VS Express for Desktop in Win8) and requires Windows/working outside of MonoDevelop. ;) On the plus side, it's a really good GUI RAD.
2.) What GUI framework is best for writing once and running everywhere?
Gtk#. Unfortunately, Mono's Windows.Forms doesn't work well and is a dead end tech. See here for a similar question.
As I mention in an answer there, if you want to write a utility that targets multiple OSes in Mono (or you're Linux-centric), you probably do want Gtk#. If you want to create a traditional client app, prepare to rewrite your GUI on each target platform.
Edit: Less nepotistic question on a similar subject here.
Edit2: There is a Mono WinForms designer that I forgot about here, but its description readily admits that, "Unfortunately the designer is not ready yet for proper use," and the blocking bug they refer to is from 2007, so not much help.
Good luck!