Unfortunately, there is no one standard GUI tool for Python. I suggest that you choose one of the above tools depending on your situation. The first factor is whether you are willing to pay to use any of the GUI tools. The second factor is whether you want the program to run on Linux or Windows or both. The third factor is whether you are a KDE or GNOME user on Linux.
I am contemplating writing 1 or 2 chapters for this book on GUI Programming. I will be probably be choosing wxPython as the choice of toolkit. If you would like to present your views on the subject, please join the byte-of-python mailing list [http://lists.ibiblio.org/mailman/listinfo/byte-of-python] where readers discuss with me on what improvements can be made to the book.