My primary image editing software is Photoshop, primarily because that's what I know the best. I also have Lightroom, as part of the same subscription package, but I seldom use it because I find its file handling model to be quite clunky and obstructive.
GIMP has progressed to the point where it is finally becoming a worthwhile alternative, for those who don't care to pay Adobe every month for the privilege of using their software. Like Photoshop, it has quite a learning curve, but there are plentiful tutorial resources available on the web, especially on Youtube.
Affinity Photo, from Serif, is another excellent Photoshop alternative. It's not free, but it is relatively cheap, and they seem to have fairly regular sales as well. It does have a decent range of image creation (i.e. painting) tools, but its main usefulness to me is in image adjustment — tone, colour, effects, size etc.
Krita is a superb free paint program which I highly recommend if you want something to draw and paint with. It also includes some image adjustment tools, but its primary focus is image creation, so it's not so useful for photographic post-processing. Its nearest non-free equivalent would be Painter.