I use Infinite Painter I'm conflicted about recommending it though because it has a habit of crashing/corrupting the files you're working on every now and then, but as long as you don't spend too long on one project I think you'll be fine and it's free!
My favorite app you pay for is Clip Studio Paint (but this has a high learning curve if you don't already do digital art a lot), my favorite free one is FireAlpaca. FireAlpaca is nice because it's a simple user interface and doesn't paywall helpful features whatsoever, and can save in other formats (such as .psd).
On my iPad I draw on a free app called iArtBook, it is very amazing for full drawings or even just sketches. It also has animation and photo editing. On my fire tablet I use another free app called ArtFlow, it's honestly good for the Amazon app store lol. I don't know any more for any other devices.
I'm conflicted about recommending it though because it has a habit of crashing/corrupting the files you're working on every now and then, but as long as you don't spend too long on one project I think you'll be fine 
