Procreate is one of the best iOS App for Artists. Procreate has a lot of tools you need to create awesome illustrations. The UI is perfect and usability frendly. Available from the App Store for $5.99.
Inspire Pro has one of the fastest and most realistic rendering engines on the App Store. With Apple Pencil and iPad Pro, the size and opacity of brush strokes react instantly to applied pressure, tilt angle and orientation for unprecedented creative control on a mobile device. (cover by Thierry Schiel)
Sketchbook Pro is a allround talent. You can create sketches with a lots of tools and the performance is strong. Sketchbook is made for young and old sketch lovers . Available from the App Store for FREE. Membership for 4.99 $ / Month or 29.99 $ / Year