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.
ibis Paint X - Drawn with + ibisPaint
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
FireAlpaca - FireAlpaca is the image editor that can be used readily and easily.
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
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.