I don't use a map application specific program, rather a standard desktop publishing program to do all my professional maps, map objects, map tiles, even RPG print/PDF document design and export. The program I use is powerful and easy to use, but capable of doing any kind of graphics job, not just game maps and tiles. Xara Photo & Graphic Designer 9 is the application I use ($89.00).
While there are many other solutions, some free like GIMP or Inkscape, those two apps have a pretty steep learning curve, Xara only a slight learning curve. But there are many possible solutions.
Edit: in the end there is no best software. I wrote an article for GMS magazine that I wrote to help you decide which software is best for you, called: The Cartographer's Table: which mapping software is best for you? (which reminds me, I need to write another article).