You should say what you want.
For 2D art i would suggest photoshop, gimp or paint.net.
Photoshop is expensive but also contains some useful things.
Gimp contains almost as much as Photoshop but is free.
Paint.net is also free but doesnt have as much features as Photoshop and gimp, thought its easier for some people.
Im using Gimp myself, mostly because Photoshop is too expensive but its still a very good program.