It really depends on which features you are looking for. If you just want a solid editor with layers and plugins then gimp is pretty good. You can also try Krita for something a little less cumbersome.
Unfortunately most of the really nice editors are closed source and only mac/windows
Krita is heavily oriented for digital painting. But it is a very solid editor too. The interface took some time to get used to. Though I like it.
But absolutely, good is subjective. It really depends on your needs. If you're looking to edit spicy memes anything could work. If you're looking for non destructive workflow GIMP and krita are starting to implement that. And if you're looking for traditional publishing specific support, good CMYK, gamut, etc. Not so much to my knowledge? If you just want familiarish.... Gimp these days can imitate the classic Photoshop interface okay.
Krita has CMYK, and very good non-destructive editing these days. It's my preferred photo editor, including for the occasional magazine ad work I do. It also has great support for PS files, including smart layers, etc, plus it has layer effects, masking, filter layers, GPU accelerated canvas, and G'MIC support covers a lot of the fancier pbotoshop stuff like content-aware fill. IMO, for the workflow and interface alone, it's leagues ahead of G***.
First choice GIMP. Then, Digikam has an image editor that provides a number of tools. Not as detailed and sophisticated as GIMP but does most things needed.
What a coincidence, I was looking at the same thing today. I kept seeing recommendations for Affinity Photo, apparently it has a wide range of features for both Windows and macOS users. It's got PSD file support, RAW support, masking, layering, retouching, blemish removal, curving, and a comprehensive set of 16-bit filters.
It's around $50 I think but you own the license forever.
EDIT : I just noticed that this is the Open Source community so I just wanted to mention that Affinity is clearly not that.
Unless you're incredibly deep into technical functions, Krita is 100% the way to go. Gimp is not horrible, but I have a bias against it, because i found it ridiculously unintuitive and hard to learn to use
If you're really used to photoshop and don't mind using a webapp I recommend photopea.com
The UI is super easy to pick up if you're used to photoshop, and for my purposes it has all the functionality photoshop has. Only downside is it's a webapp with no local client available as far as I know.
As Gimp 3.0 is nearly done, Photogimp likely needs an update as its made for GTK2.
GIMP 3 introduces some necessary features that are a base requirement for graphics design, nondestructive filters and color profiles (if you want to print)
If you just want something that's free as in price, honestly the best thing (in terms of functionality only) is pirated photoshop natively on windows. I've found PS is kinda crap on Wine.
Photopea is proprietary so at that point you may as well just pirate photoshop, at least then you're getting the real thing.