Whatever I feel like.
I've come to the realization that it doesn't really matter at all. As long as it has basic tools (undo, layers, pressure sensitivity, lasso selection tool), I can pretty much use anything for more or less same results. I used to be very picky about the line quality and inane shit like that so I had hard time finding a good program, but somehow I learned to look past that.
I juggle between FireAlpaca, Manga Studio, Krita, and Photoshop.
I primarily use Photoshop though because it does everything well and I'm the most familiar with it, and you can do some neat tricks in it. The reason I'm not settling with one program is because it's fun to do different things sometimes, helps keep things fresh.