Best framework to build a GUI app in Python for windows and macOS?

It depends on what your goals and constraints are.

If you want to write a "traditional" desktop app, I would say the most powerful framework is Qt (you can use either PyQt or PySide). But there is a substantial learning curve as the framework is huge and takes some time to come to grips with.

If you just want to do something simple, Tkinter has the advantage of being included in the Python stdlib.

I think the best one right now for python is "beeware": https://beeware.org/

You also have Kivy which is prety good: https://kivy.org/

I doubt that it's the "best" in many ways but it's certainly convenient and interesting to use Streamlit as long as it's not too complex and fits into the framework.