What Intel Motherboard is used most often in business-use personal computers?

Do you mean recent models or all computers still in use? If the later,
then my guess would be Intel CA810 - Intel motherboards were more
popular then, and CA810 was very widespread. Most computers from that
time are still in use now. This is based just on my experience though,
I do not have any facts to back it up.

If you mean recent models, it still will be something with integrated
video, probably the Intel D845GV - business use usually doesn't need
powerful graphics. The D865GV wasn't so widespread since it was more
expensive, and the 915 chipset boards are too recent to have achieved
a large market share.

I think this really is the answer to my question. I want to
standardize on an Intel MB for a new business computer model I will
build and sell and wanted to know which one has proved to be most
popular and reliable. Thanks!