The basic version of Autodesk's SketchBook app has been in the Play Store for a long time, but now the professional version has come to Android. Autodesk SketchBook has more advanced tools, a ton of brushes, layer support, and a lot of it can be accessed for free. Of course, the best stuff is behind a paywall, but it's not too expensive.
The desktop SketchBook program is aimed at illustrators and artists and costs $65 for a full license. The mobile app isn't quite as full featured, but it has more going on than most of the competition in the Play Store. The free version of SketchBook includes customizable brushes, synthetic pressure sensitivity, and Dropbox integration. It also does palm rejection in pen mode. Signing up for a free SketchBook account adds the layer editor, symmetry tools, and more brush settings.
The full app can be unlocked for $3.99, but anyone paying for the desktop version gets it all free by logging in. This version includes over 100 brush presets, advanced layer control, selection tools, and plenty more.