Well, I am massively biased in this regard, but I can and most likely always will say that Photoshop is pretty much the best mapping tool one can hope for. The possibilities it offers are just limitless, and there are a ton of great tutorials on this forum on how to make maps with it. It's not the cheapest, but it will enable you to do whatever you want once you get the hang of it. I went in knowing nothing about it, and the tutorials here have taught me to both use it effectively and to find new and cool ways of making things.
Of course it depends on what you want. GIMP is another software that a lot of our people use and can make awesome maps with it, and it's free.
Krita I believe is a free drawing/art program that a lot our mapping people have gotten into using and I've heard good things about.
Affinity Photo is another photoediting software that is said to be a serious contender to Photoshop, but it's amazingly cheap compared to it so it might be worth checking out. And then there is of course the mapping software Campaign Cartographer that a lot of people use as well.
All of them can produce good things, but it all depends on what kind of look you are going for.
You might also want to consider using a vector based program rather than a raster one, if you want to keep things relatively simple looking. The upside of vector graphics is that they scale infinitely, so you don't have to worry about the resolution of your maps. Of such programs, Adobe has Illlustrator, then there is the Affinity Designer that is very cheap and I personally like a lot, and I believe Inkspace is a completely free alternative to these.
In the end, I'd say it depends on the level of your need to control everything I prefer PS because it allows me to create everything from scratch. It takes more time than importing ready assets made by someone else, but I can lay out every. single. line. the be just the way I want it and not one pixel less.