I use MyPaint, a free program. It's a bit odd, can be laggy, and definitely has a learning curve. But I find it to be more like drawing traditionally than other programs, making it much more comfortable to use. It also has some nice brushes, too. However, it lacks many features that PS/Gimp/SAI have. You can use the arrow keys to move around the canvas, and the mouse wheel to zoom in and out, which is easier (IMO) than using scroll bars to move around and having to keep adjusting the zoom via a menu.
Photoshop for sure. I've used paint tool SAI before but I just can't get used to sketching with it, I guess. It's great for very refined line art, though.
I use photoshop CS5 and sometimes illustrator when I need something vectored but mostly photoshop. I've tried Painter SAI just briefly and I found it too complicated especially when im not an expert on digital paintings but quite a good program to be honest, takes alot of processing power though
So, which is you favorite? and why?