Adobe photoshop? Photoshop's too ridiculously expensive. I tried out a whole bunch of stuff and found Manga Studio 5 to be a drawing program that makes me happy (and that didn't cost a small fortune - I was able to pick it up on a special for about 25USD).
It doesn't have as many features as photoshop or all the effects and resources, but it's really nice to draw in - I get good feedback and things are more likely to come out as they would if I'd drawn them on paper (this is a problem in some of the cheaper software). So I'm pretty happy with it.
