Darktable if you want to do professional photography editing, and GIMP if you want to modify graphics, pixelart, design flyers or handouts, posters, or whatever else you want to do in a raster editor.
I have been using GIMP since fifth grade in school when we had an intro to photo manipulation, and since whenever I needed something edited, composited or designed. It blows my mind again and again that people online apparently find GIMP hard to use or unintuitive. It’s one of the most normal programs I have ever used. Photoshop on the other hand feels utterly inconceivable to me.