Exactly which design software package for landscape planning is best for your needs will come down to a number of factors, some of which are listed below:
Freeware.
Commercial.
Which Operating System platform you run.
How much time you're planning, willing to invest in learning the software - some of these packages are extremely powerful (and complex once you get below the surface of them).
Do you require an API.
Are you interested in a strong user community (eg User help/support).
Whatever package you decide to use, I can't recommend strongly enough that it be vector based.
My first recommendations are "simple" (and free) CAD/Drawing packages, that's not to say they're not powerful, just can be viewed as somewhat limited by (some) "professionals" in the design community:
Personally I would go with Google SketchUp, as it's:
Simple to use.
Large user community (eg Lots of freely available content, tutorials, support, etc).
2D & 3D.
Allows for an easy upgrade path to Google SketchUp Pro if required further down the road.
My second recommendations are "powerful" (commercial) CAD/Drawing packages, bias to Autodesk products, as that is where my somewhat expertise lays:
It's a difficult one to recommend, but for landscape planning I would probably have to go with AutoCAD Map 3D.
My third recommendations are "powerful" (commercial) non-Autodesk CAD/Drawing packages, though to be honest I can't really recommended any of them as I don't have any "real" experience with them, I'm aware of them, have very! limited experience with a couple of them:
For the "average" homeowner that wishes to do some, get into, landscape design, I would have to recommend starting with Google SktechUp.
Personally I would use Autodesk products, but that's because I have 15 years experience with them...