I think it kind of depends on what you mean by a galaxy map. Isomage is scripting up a Traveller sector map. I did a scripted galaxy part map for the challenge a while back (Aug08 maybe...) and many programs can use tokens and CAD type effects to make galaxy maps by hand. I think there are plenty of options tho if one style of map floats your boat then I reckon that only then is there a 'best' app for it.