Guess which insurer denies the most claims?

The government.

"The Medicare denial rate found in the study was, on a weighted average basis, roughly 1.7 times that of all of the private carriers combined (99,025 divided by 2,447,216 is 4.05%; 6.85% divided by 4.05% =1.69)." Read the whole article here. The research used the AMA's 2008 report.