Which maps you need depends on what you want to do with your unit.
You will need two components. An application and a source of maps.
For in-vehicle routing and navigation then you will want something like TomTom - which combines maps and s/w in one package.
If you are hiking / going-off road then you need a topological program. Here in the UK we have Anquet maps with excellent coverage of the UK at 1:25000 (very expensive) or 1:50000 (much more affordable). For you in the US I can't really suggest anything as I don't know that marketplace. But....
If you have lots of paper maps - or access to digital mapping (which I seem to recall is more affordable - sometimes free! - in the US) you may want to take a look at OZIExplorer / OZIExplorerCE (
www.oziexplorer.com) it needs more work by you the user - but it really is a very nice piece of software. I'm not associated with them in any way but I have been a very happy user of the software from its early days in the 90's.