Over the past 18 years of ownership, I've been through AOPA Insurance Agency, Avemco, AIR-Pros, Travers, Falcon, and a couple others. So far, these guys are outperforming all of them by a wide margin with regard to response time and ease of doing business:
http://www.wings-insurance.com
With the exception of Avemco, the agencies all work with basically the same underwriters, and quote the same premiums. What I found over the years was that brokers are eager for new business, but after the first year, their responsiveness drops off significantly. Some brokers are still messing around with paper forms (I dislike paper). Some brokers are lazy and won't shop the markets every year. Some brokers "forget" about your renewal until a few days before the expiration of your current policy.
All of the agencies mentioned above have been good in some ways, but I haven't found them to be consistently good--except for Wings. None of these guys are making much money from recreational piston airplane pilots, so I can understand how we end up at the bottom of the priority list, but I think that most of the problems I've encountered have been related to business process more than anything else. Wings seems to have their act together all the time.
JKG