Or the least?
Washington's state insurance commissioner's office has built an online tool that allows you to easily compare complaint rates by insurer.
The results include a "complaint index," which compares the number of complaints to the company's market share. You can also compare the complaint rate based on the total premiums charged here in Washington. The data's broken down by insurance type: auto, homeowners and health. We've got four years' worth of data built into the site (calendar years 2005-2008), so you can also look at who's improving or getting worse.
Inevitably, of course, there's a caveat: Be sure to also look at the total number of complaints. For insurers with a very small number of policies here, even a single can push the complaint index quite high.
Click here to go to the "Complaint Comparison Tool."