I have the following and enjoy them all for different reasons. They are not presented in any specific order. I've included my thoughts as these apps pertain to painting. I hope this helps.
EDIT: No matter which app you're considering - buy a nice stylus. My work quality increased immediately after getting one.
ArtRage
Probably my favorite sketchbook app. It has the widest selection of realistic medium and surface textures and has a very realistic blending of the mediums. The interface is pretty easy to use and it gets out of the way when you're working. After the last update I believe you can have the image re-painted by the desktop application (Artrage Studio Pro) at higher resolution, but I haven't tried it yet. The only negative is that it can be pretty resource intensive on the iPad 1. It doesn't crash too often, but it can get laggy, especially when using a trace image. The last update helped, but I recommend a reboot on if you're going to be working for a while.
Procreate
I just bought this one and haven't used it much compared to the others, but thus far I'm very impressed. It has been very responsive and has nice (customizable) brushes and blending. It's not quite as realistic (in terms of traditional medium) as ArtRage, but can yield some very nice photo-real paintings rivaling what you would get working in Photoshop. Sadly, it seems to have a limited resolution on iPad 1.
ArtStudio
This is more of a Photoshop for iPad. It has layers, a load of tools including pencils, brushes, text, smudge, marquees, clone etc. It's winning features for me are layer blending modes and effects filters. I use this as a finishing app most often, but you can easily sketch & smudge with it. It's a great all around app, especially if you want something that can also edit photos.
Brushes
It's been around forever and was the standard by which others were judged. I got this and Sketchbook at the same time the day I bought my iPad because they were the two heavy hitters with the most features. The thing that drew me to this was being able to replay the paintings at a higher resolution on a Mac. It's easy to use and gives great results. The paintings are obviously digital, but you can get a nice level of realism and style from it.
Sketchbook
I've been a desktop user of Autidesk Sketchbook for years. The interface is fairly easy to use and very minimalist. The mirroring, text and free transform tools made this app stand out. I ended up using Brushes more because the resolution is limited on the iPad 1.
Inspire Pro
I grabbed this because it was free (on sale) and was pleasantly surprised with the speed of the strokes and the blending was great. I never did any serious work in it and once ArtRage came out, I stopped using it because of ArtRage's realism. It's not exactly lacking by any means and can hold it's own with the others on this list. There were a lot of nice surprises like being able to rotate the smudge blade and the immense help and tip system built in.
