LIke anybody it depends on what kind of software you like to use - do you prefer vector (like Illustrator or Inkscape), do you prefer an image editor (like Photoshop or GIMP). It depends upon your budget too. There really is no one best software. I use Xara Photo & Graphic Designer, a vector program like Illustrator, and though I primarily do fantasy maps, I also do sci-fi maps. I can easily place any kind of grid on top as I require, including hexes. You could look at my Google + page, 25 Quick & Dirty Map Tutorials, near the top and going back a few months of posts, I have several sci-fi location, a starmap and a few other sci-fi projects posted there, to give you an idea of what Xara can do.