Clip Studio Paint - The artist's software for drawing and painting.
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.
FireAlpaca - FireAlpaca is the image editor that can be used readily and easily.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
ibis Paint X - Drawn with + ibisPaint
PaintTool SAI - Learn to draw in different ways.
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.