On PC: PaintToolSAI. It's a bit 'outdated' these days, but I love it. Very lightweight, which it manages to do by throwing out anything that might be considered an "image editing" tool. You get layers, those layers can have, like, one of four effects, and you get some brushes. You can make your own brushes pretty easily. It's best if you're a cel-shader. You can do painterly art with it, but you'd probably get better results with more robust software. Its line smoothing/precision tool? Ace. The entirely reason I stick with this software and deal with it having a total lack of text and shape options.
On iPad: Procreate. Awful name (I can see what they were going for, but still), but a wonderful little one-time-purchase app. No ads or other nonsense. You kind of *really* need an Apple Pencil, or at the very least a more expensive third-party stylus though, if you want line tapering. You can kind of cheese it with touch tapering, but you have no control and the app just kind of tapers lines as it thinks is best. I use a $20 Amazon stylus, and it works well enough. It could be better.
What I don't use:
GIMP (PC): It's free. It's also got an intimidating UI and kind of a steep learning curve. I've never liked it.
FireAlpaca (PC): It's also free. It's kind of a SAI clone, with a few more options and tools. I've used it, but never got into it too much. It didn't really present me with much of a reason to switch permanently, but I saw nothing wrong with it.
Photoshop (PC and Mac): It is not free. Also, it's kind of a lie. I do use this extensively, but not for art. It's still kind of the "standard" but it's stupidly expensive unless you get the entire suite of Adobe products and use them regularly.
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