That's something you will get many answers to based on likes and dislikes. Gimp is a freeware program that works well, then there is PaintShop Pro and Photoshop (both payware) and there is one other that I used a long time ago that also allows for working with "layers", which is a must if you intend to do any kind of repainting, however the name of that program eludes me right now. If I happen to remember it I'll post it later.