What would be the best microscope for surveying the contents of a petri dish?

A stereo microscope would be effective for examining petri dishes. The stage is large enough to hold big dishes, and the wide field of view will make it easy to sweep the dish for colonies or specks of material that might be missed using a narrower field with a higher-power compound microscope. 

Switch to a compound microscope using higher powers when looking at individual colonies in the petri dish.

 

Updated 12/18/13