The best on my opinion - Corel PhotoPaint... The best for many others - Adobe PhotoShop... What's bad about them? They are paid and very expensive! Regardless that many will say "whatever I can crack it" - isn't there a free alternative? YES IT IS -
http://www.getpaint.net/ Paint.Net really grabbed my attention and won my vote for first place on quality and usability in graphics editing.. It's very small, very fast and have everything that I want... Finally - it's totally free! I LOVE IT