[Lain]

Paint.NET is BASED but unfortunately it's not open source so by default it infringes on my rights and beliefs.
GIMP is okay if you're doing actual photo editing like anything regarding filters or manipulation of an existing image, but even their own docs say that it's not for drawing: https://docs.gimp.org/en/gimp-using-rectangular.html

For drawing, they recommend Inkscape but that's too oriented for vector graphics (i.e. it's an Adobe Illustrator alternative instead of a Photoshop alternative)
Personally, the best opensource free paint software I've come across is Krita. The UI is gorgeous, has like 90% of the same features Photoshop has, is more lightweight in terms of CPU and RAM usage (not to mention install size) and works perfectly cross-platform for all the Windows kids.

But I just use Clip Paint Studio since I got it on sale for 25$ (every three months or so) and it's more oriented towards drawing animays and their community resource repository for brushes and whatnot is f*** HUGE (not to mention free 10GB cloud storage for your assets and drawings)