Entirely depends on the project you want to build

Pandas. Python's only killer library imo

That's like saying "what's the best ingredients to learn cooking with?", firstly it all depends on what your want to eat, secondly it doesn't really matter what the ingredients are to learn cooking skills.

Flask is a fun framework for making web apis.

Pika is a client for RabbitMQ, if you want to try message queue stuff.

Numpy and sklearn for numerical and machine learning stuff.

Matplotlib for making nice plots of your numerical stuff.

Pytorch for deep learning.

Pillow for image processing.

OpenCV for computer vision.

Pygame for 2D games (maybe a bit old, but I had lots it fun with it when I started learning programming years ago)

Standard library of course. And collections module too, among the others that come by default. What you need is assignments to solve. Like project euler or hacker rank.

If you want to do web requests/ use API's, use 'requests'

graphs/reporting, I've used 'bokeh' before, it was nice.

I've never used PyDroid, so I'm not sure how you'd install things, but these are both available via pypi, python's package repository.

I heard there are quantum computing libraries in Python if that interests you!

If I were you I'd browse PyPi for any packages that look cool.