Another vote for Hagerty. Beginning with my 1996 Bentley Turbo RL, which various companies would insure, Hagerty had the lowest premium compared to a half dozen other companies, and they used agreed-upon value.
When it came time to insure the 2006 Flying Spur, Hagerty was the only company that would insure it. Same happened with the 2014 Speed.
The only thing which might be an issue for some customers is that on all three of those cars the premium had to be paid in one lump sum for one year in advance. I'm not sure if that was a condition just on my policy for some reason (e.g., advanced age of driver, high value car) or if that is true for all such policies at Hagerty.
Another feature I like about Hagerty is that when I call the national office, I quickly get to a friendly rep.
Hope this helps.