Procreate - drawing and sketching
Having just bought an Apple Pencil to play around with sketching and drawing using Procreate I can certainly see what makes the iPad Pro a pro-level tool. This is really impressively awesome stuff and the experience with the pencil is very fluid and convincing.
I'm just beginning to dabble with it, but for me the hardest part so far is the complete lack of friction between the pencil and the glass screen. It makes even the simplest things - like drawing a circle - a lot harder than I'm used to on real paper. I hope it doesn't take too long to get used to that