As far as I can tell, PanGUI is a drawing library not a graphical toolkit. Its primitives are geometrical, not widgets. Its showcase is an audio app, which is as far away as possible from a boring productivity application that I'd like to do.

In my university days I was very much into GUIs, and I've written apps with wxWidgets, plain Gtk 1 and 2, GNOME 2, Qt, Tk, GNUstep and even some fairly obscure ones like E17 and FTLK. For my tastes, the nicest ones were probably GNOME2, Elementary and wxWidgets. Especially GNOME2, which had a simple builder that let me create the basic shell of an app, with some horizontal and vertical layout boxes that I could later "hydrate" with the application logic.