I think Scribbles is best in show EXCEPT for one very annoying thing - the jagged edges on the brush strokes:
If you zoom to default zoom level and draw some strokes with the smallest possible brush size, you'll see horrendously jagged edges on the strokes, both in the application and in the final exported image files (no matter what export format you use).
Sure, if you zoom in on the strokes you've made, the edges go smooth as butter, but again, if you zoom out to default or even further out/away from the strokes, you'll see these jagged edges.
For me, this is the only downside with this app, alas, a flaw that makes me not using the app.