The three big players I know of are Hagerty, Grundys, and American Collectors. I have used American Collectors since 1999 because they are affiliated with USAA, which is my insurance carrier for my daily driver, home, and life. Last week I wanted to see if Hagerty or Grundys were cheaper, so I did online quotes. They were all about the same price for identical coverage. I believe they all offer full reimbursement on agreed value of the car, but you need to double check that for yourself.
My policy limits me to 2500 miles per year and the vehicle can't be used for transporation to work, but AC offers policies with higher mileage tiers and occasional daily driver usage.