I only offer Hagerty. I have access to American Collectors, Philly, Etc but Hagerty does such a good job with claims I won't sell anybody else. Most of my classic car clients are also business owners/commercial clients so I won't risk a big commercial account for them to save $50 in premium on a classic car policy.
I'd recommend Hagerty as well... they are some of the most reliable classic car insurers. I don't have much experience with the others, but I know several people that really like Hagerty.