@Hubertrum: Glade is not a GUI but a GUI designer for the Gtk GUI toolkit.
Asking for the best GUI toolkit is like asking for the best programming language — you will get different answers from different people about *their* preferences. That's why you can't find a discussion with *the* answer to that question.
It is not a RasPi specific question because you can use all the GUI toolkits available from the Linux distribution you are using. Don't download tar files from the internet if there are packages in the package management of your Linux distribution! Python bindings for `Tk`, `Gtk`, and `Qt` should be there to install.
`Tk` is somewhat „old” and has not that many widget types „built in” as the other two. There is no widely used graphical GUI designer, but I never saw that as a negative thing. `Gtk` and `Qt` are actively developed and alive. There are `Glade` and `Qt Creator` for graphical GUI design.