I owned and tried several, here's a few(depending on device and my personal assumptions):
Free and beginner friendly:
Autodesk Sketchbook -minimal UI, but may be device dependent, good for learning how to use a digital drawing pen if you have one.
*IbisPaint(free-mobile?) - my first digital software on a portable tablet, a step up from Sketchbook and Paint, I assume as far as more features to play with. Worth paying for X if you like it and want to take it a "step up"
*Krita(PC/hybrid) - my second software, on a hybrid tablet/PC
-lots of editing features/filters
-good to learn how to work with layers and more advanced editing/filters
-animation is intuitive/easy to learn
-recommend if you like painting and/or drawing in general but don't want to pay for anything too serious.
Paid:
(only recommend if you tried everything and/or want to take your work to more advanced level)
*CSP(PC/tablet?/mobile?)- better for more advanced artists, and very webcomic-friendly, my current fav
-advanced vector-based functions (vector eraser is a life saver for me, literally the reason I got it-with a discount)
-animation can be a little difficult to learn(weird folder/keyframe system), but it was made for more traditional anim versus computer-assisted anim (like Blender)
-there's a free 3-month trial, unless they got rid of it in recent update (I have 1.0 perpetual license still and don't update, frankly, until after a few years/versions cause I'm slow to change and dont wanna be scammed)
(Sorry if it's a lot of info, this is coming from 5-6 years of experimenting and a prior 3d animator wannabe.)