Clip Studio Paint - Medium Cost, Advanced, very powerful, multiplatform.
For me doing illustration that is mostly line art, nothing beats Clip Studio Paint.
There is just so much sophistication in this app. I really think the people who make it truly understand the work that goes into making an illustration at a very deep level.
And it shows up in for example all the very interesting options they have for filling an area with the color, or how they incorporate vector based reference layers.
I don't think there's any other program out there that is as productive for doing line art - for example things like comics or manga - as clip studio paint.
Although clip studio paint is not free, it does not cost that much either. I subscribe to a plan where I pay about $52 a year. And that lets me run clip studio paint on four separate devices