Procreate is probably the best. They seem to be very serious about creating a powerful painting app and keeping it up to date. SketchClub is another good app that's being kept up to date. Sketchbook Pro is awesome. I wouldn't touch ArtRage with a ten foot stylus; it's really slow and laggy on the iPad 3G, though it might be a bit better on the iPad 4G. Brushes 2 is a really good, simple app that has the advantage of recording every brush stroke so your entire painting can be played back as a movie, uploaded to YouTube. Unfortunately, rather than updating the app, the creator of it seems to have rewritten the whole thing and called it Brushes 3, which sucks compared to the second version. If you can find the older one in the AppStore it's worth the money.
On another note, when it comes to styluses ( which to me I can't see anyone doing without if they're creating art on iPad ) I would recommend the Bamboo ones. I've used mine for over a year and am very happy with it.
I have all the big name art apps on my iPad, and I will just say Procreate, Procreate, Procreate ( the name of the app of course! ). It's got great workflow, tons of brushes, loads very fast, let's you create in HD etc. Go with Procreate.