I've used ArtRage, Corel Painter, PaintstormStudio, Rebelle 2, Adobe Sketch.
ArtRage is fairly decent, available on both iMac and iPad, Wacom tablets and Cintiq's and PC.
Corel Painter on iMac and PC, Wacom tablets and Cintiq's, I think it has a fairly steep learning curve, plus if you want additional brushes you have to purchase them.
PaintstormStudio is my favorite (used to be ArtRage) and it does have a huge learning curve, but it comes with all the bells and whistles and then some...iMac and iPad, Wacom tablets and Cintiq's, not sure about PC.
Rebelle 2 is a great program for watercolor, but it's iMac and Wacom Cintiq's, not available for the iPad, not sure about PC.
Adobe Sketch has a decent watercolor app for iPads, not sure about other tablets or PC's.