On windows, i used to love Photofiltre.
However, i have since switched to Devuan and i now use GIMP.
Some people recommended me Krita, however i find it very unsuitable for pixel art & copy-pasta, which GIMP excels.
I don't like some of the changes GIMP made in 2.10 though... Maybe i'll have to complain the s*** out of them in the bug tracker.
I use GIMP when what I am doing becomes too complicated. I would like something in the middle, where the advanced features are *there* but you don't *have* to use them. Sort of like why pointers suck in C, IMHO. (They are there and you *have* to use them