ProCreate (according to my girlfriend, who has a degree in illustration) is absolutely second to none with regards to completely eliminating the friction from digital painting.
The toolset & UI/UX feel like someone who had used Photoshop with a Wacom tablet for 10 years took everything they liked about interacting with Photoshop and got it right, and everything they hated about Photoshop and fixed it.
The app itself is absurdly responsive on a 2018 iPad Pro. I don't know if you saw the Hobbit in the theatres when everyone was freaking out about the frame rate because it felt too fast? It's like that, but enjoyable. Every other app on my iPad seems sluggish in comparison, and I don't even know how. (And don't get me started with comparing it to Photoshop, even on a ridiculously beefy machine with a 144Hz display.)