What is the best program to create GUI for Microsoft and Mac

gimp

if mockups then pencil

No, not mockups the real thing. I'm using QtCreator right now. But i want to check to make sure there isn't something better before I jump in

I think you're referring to GUI libraries not how to create them. Qt's editor has a nice built-in builder for GUIs if that's what you're looking for. NetBeans GUI builder saves me a Hella ton of time.

I haven't really found a GUI creator that I actually really like. Most of the time what generate is really bulky and inefficient. I spend more time coding around it then actually doing stuff until I say screw it and turn around and just write it all by hand.

Gtk# is pretty good. Very similar to Windows Forms, runs with Mono .NET.