The best landscape architecture software is what you can get the most out of it. I was with a firm early in my career that had 3D Studio, but nobody in the office knew how to use it beyond beginner level!! Its like keeping a Hummer H1 vehicle in your garage for only a 2 mile, grocery store trip on paved roads. In order:
AutoCad
MS Word
Photoshop
MS Excel
MS Powerpoint
Sketch Up
Basic GIS / ArcView
The rest is icing on the cake!!