David Hockney. I'm not that far along the trail, but I've been starting to feel my way through.
I've beem using ArtStudio. Hockney has said he uses several different programs, but most commonly Brushes. ArtStudio doesn't even make this list of the best programs, and maybe I'll experiment with a couple of others, but no one would list Microsoft Paint among the best programs for PC, and it's the one I use most.
The nice thing about iPad apps, as opposed to computer programs, is that they're cheap.