Thanks Brian
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I use a Nikon 105mm VR Macro lens on a Nikon D300 so unfortunately not an option for your 1D MkII. However I know a lot of members use Sigma lenses and have been very pleased with their results.
The main consideration is probably which focal length to opt for. With Sigma the options are (I believe) 70mm, 105mm, 150mm or 180mm. They all offer ~1:1 magnification so probably the most important difference is working distance and cost. Essentially the longer the focal length the greater the working distance (an advantage with skittish insects). On the other hand the longer focal length lenses are heavier and they cost considerably more.
Your 1D MkII sensor is full frame so will influence the compatibility and performance of a macro lens with DX coverage. Maybe it would be worthwhile geting recommendations and info by posting a new forum thread.
There was a recent forum thread asking for recommendations for a macro lens that you might find interesting:
what macro lens??
Bruce