For large-scale maps, a combination of Inkscape, Gimp, and Wilbur for me! The general process is:
- Rough out landmasses in Inkscape
- Rough out a heightmap on those landmasses in Gimp
- Simulate erosion on Wilbur
- Bring it back to Gimp for colouring and manual details
- Back to Inkscape for labelling
I made this as a fancied-up version of a Quiet Year game I played with some friends https://i.imgur.com/gB3sIEs.jpeg
For smaller-scale ones like D&D battle maps, just plain old Gimp (or sometimes some layout with Inkscape beforehand, and also sometimes some textures made in Inkscape). I made this version of Biete Mariam, a church in Ethiopia https://i.imgur.com/nz1ysvY.jpeg