DynaScape or Landscape Pro. DynaScape is ALOT more powerful and ALOT more costly with a price tag in the thousands. Also alot more complained BUT it can be used by LAs and LDs. Landscape Pro I think is the best for landscape designers and I use it. It pays for itself.
I was trained on Dynascape while I was at Virginia Tech. Back then I would of said it was the best but Landscape Pro has come long way. Just simpler and best solution for a landscaper that isn't solely designing. All the above guys are very right. It seems like a lot of money up front but it will pay for its self VERY quickly. Do not waste money on the cheap stuff out there, all you are doing is wasting money.