It depends on what I'm trying to do. If I'm just trying to cut apart image strips, I use xpaint. If I'm trying to do some major image manipulating, I use Gimp. And there is also the rendering software I use. Incase you are wondering I'm using all this on Linux therefore I dont have the ..."pleasure"... of MSPaint (sorry Game a Gogo)
Yes, it depends in what one has to do. I already wrote it in another thread: Personnally I prefer Corel Photopaint (probably because I am most used with) but Corel 11 does not support png transparency. So I have to use Adobe Photoshop for my animation frames.
I heard that Corel 12 is released. Maybe they support PNG transparency NOW. (About time)