I have worked with many...

Best advice:
Buy the best you can afford, because the best is always the OEM tool.

Here is a list of several good scan tools.

The Mercedes STAR tool is roughly $20,000.00 USD, with a one year subscription.

http://www.google.com/search?hl=en&lr=&q=obd2+dtc+scan+tool&btnG=Search

I have thought to buy the OEM VW AUDI scan tool several times, however the cost is high...

http://www.google.com/search?hl=en&lr=&q=VW+factory+obd2+dtc+scan+tool&btnG=Search