I've tried and/or purchased almost every painting app under the sun. But I keep coming back to Artrage. Artrage is simply better for its ease of usability as a pure illustration app. You can find all of Artrage's features in other apps (and even features that Artrage does not possess), but rarely will you find them as unified and accessible.

One feature that I really like with Artrage is the post-process on strokes (smoothing). I have a jittery hand and cannot draw smooth lines. Other apps tend to handle this by using a 'drag tail', or does a sloppy post-process (Clip Studio). This is one of the main reasons I keep coming back to Artrage.