[bemused]

"the best Linux desktop toolkit" depends primarily on the language you choose and the quality of its bindings to the toolkit

but consider that Qt aims to blend nicely into a GTK-based desktop environment, while its sometimes not even possible the other way round - eg using the "native" widgets of the desktop environment like file dialogs etc

"cross desktop environment" or whatever you might call this, is definitely not a top priority for the gtk-devs (see https://bugzilla.gnome.org/show_bug.cgi?id=735211 for example)

its all fine if you develop for GNOME only, but you should be aware of GTKs current limitations when it comes to writing applications that look nice on all linux desktops

and porting from one toolkit to another can be heaps of work, so choose wisely..