Mypaint is an excellent drawing application, huge canvas, the interface gets out of the way, lots of great media, pens, brushes, charcoal, etc.

I just ran

sudo apt-get install mypaint

and it installed in seconds :D – I love the ubuntu repos <3

The occasional crash is fine as I save my work all the time and just reopen it.

I have tried drawing with GIMP which is OK but more for Photo editing – so it very good at adjusting lighting in a digital painting – using layers and the dodge and burn tools

Free software is great for teachers and students as you can give away copies with a class.

Krita ran too slow – it looks great just like painter but it runs slow for me and a responsive interface is essentiial.