Brushes

Brushes was pretty much the firestarter for me in 2009. Compared to all other apps it's the strongest in trying to be not emulative, but instead building on the strengths and differences in using data as your material. It was the first to save all your behavioral data in a scripting files that allows you to replay and export your process as a movie, or re-paint your piece up to 6 times the original size.

SketchBookPro/Mobile

SketchBookPro and SketchBookMobile were the second apps that came out inspiring me a lot. SketchBook has a great variety of brush tips, that all look very professional and crisp. You can see the apps past as a desktop app since it has inherited a lot of best practices in terms of functionality and interaction (e.g. if you setup all the gestures to your desire, you can flow thorugh main functions pretty quick).

When procreate hit the app-store, I was blown away by one thing: its speed. Since its painting engine is complete based on openGL, it can take full advantage of the GPU instead of clogging the CPU. Add a polished interface, the possibility to create your own brushes and a freely rotatable canvas: what you get is one of the best painting apps out there. Plus it's indie-software, and the folks at SI are quite responsive.

ArtRage

Like SketchBook, ArtRage has a past (and a present) as a desktop app. ArtRage ist to my knowledge the only emulative app so far, meaning it doesn't power-clone stamps, but aims to emulate behaviour of material, like oil, pencils or different paper textures. That said, you can tell it's an app that pushes the hardware limits of the devices and be a little slow at times. Nevertheless the feature scope you get is amazing, including the possibility to pin a photo to your canvas to paint from, or even tracing a photo.

Eazel

To be clear, Adobe Eazel is, at this point, dead in the water. It hasn't seen any major updates since its initial release. It is also a quite controverse app, since it doesn't spot any point&tap iteractions. Instead it's based on a 5-finger interaction, which many people find awkward. Personally, I still find it a fascinating approach, but what I like most is the data-driven visual quality of the paintings. It aims to simulate a 'drying-process' without actually emulating the looks of watercolor, instead what you get is more of a particle-simulation. The rsult is a new aesthetic quality.