Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
Clip Studio Paint - The artist's software for drawing and painting.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
ibis Paint X - Drawn with + ibisPaint
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
FireAlpaca - FireAlpaca is the image editor that can be used readily and easily.
Being an architecture student, Open Canvas has been useful to me in creating perspective sketches for buildings and design concepts, owing to the ruler or grid feature.
I recommend - openCanvas.