SketchTime (App-Store-Link) by Hansol Huh is one of those do-one-thing-and-do-it-right apps. Its purpose is to mainly let you sketch a piece that you can build upon later in a different app. I love to use it a main tool, and often drop it into a filter app for a certain finish.
PaintBook hasn't been updates for at least a year now, but it's a hidden gem. If you manage to get past the awkward interface and interaction, you'll get an unmatched vector painting tool, that allows for freehand action without the often annoying smooting you get when using vectors. It also exports to pdf, so you can even do large-scale printable poster-artowrk with it.
After Brushes, Inkpad was the next app developed by Steve Sprang / Taptrix, best described as FreeHand (not Illustrator!) for the iPad. Simple and powerful tools, with witty gesture support. I think there's a range of apps like this one out, that have more features, but for my needs, this one hits the sweet spot.