My best rental car insurance experience was with Allianz via a Barclays CC. Yes, it was that Allianz with seriously tarnished reputation following WW2. But their handling my car accident in South Africa ~ 6 yrs ago was just a pleasure.
A much more recent experience with AmEx Insurance from last year was rather horrible. It was just a small scratch (also incurred overseas), but they wasted enough time to repaint a hundred cars, and ultimately underpaid the charge to my CC. I'd rather avoid AmEx Insurance from now on.