Procreate, a powerful art app for your iOS device allows you to sketch, paint and illustrate app. This app is highly rated on the app store and is an Apple design award winner. There are a massive amount of brushes, layers and the resolution is very nice. The app works very well with the Apple Pencil too.
The updated version fixes various issues.
If you are looking for a serious app to sketch, draw, paint, etc. check out Procreate.