Best is in the eye of the beholder. I like wxpython. The new phoenix version for python 3 is excellent
Tkinter is built in, and is OK, but its geometry manager is a total pain in the butt, and it's appearance
is old. Qt is loved by many, I like it as well, but stay away from it because the commercial version, I fear
that one day they will cut off community version. I've been around for some time and have been painfully burnt
in the past (not by GUI company). That's my fear, It's probably just fine.
You can visit:
https://wiki.python.org/moin/GuiProgramming and see what's available, and make your own decision.
If you'd like to see what wxpython is capable of, you can visit some of the links provided here:
https://python-forum.io/Thread-wxPython-...ep-by-step