For ourselves DW & I prefer the Delta Amex card. We like flying Delta and have for several years. We travel with friends so the companion airfares work great. The travel insurance is a nice perk and so is no foreign exchange fee.
For a backup, when Amex is not accepted, we each have an American Airlines Mastercard. Has a similar travel insurance perk but does charge a foreign exchange fee.
If you travel outside the US much, I would add a CapOne card as a backup, or if you are a member of a credit union, see if their cards have foreign usage surcharges or not. I belong to two credit unions and the cards of one do have that charge and the other does not.
Working full time in eastern Europe for a number of years, and on shorter temporary stints since then, I became very aware of the lack of acceptance of AmEx. In one country where I worked, there was only one place in the entire country that took AmEx, an upscale hotel in the capital, while Visa and MC were widely accepted all over the country. After a year or so after I started, that hotel stopped taking AmEx, so nobody in the entire country took it anymore. Two other countries I worked in were almost as bad and it was rare to find someone who took AmEx while Visa and MC were widely accepted. What was bad was that the corporate card we were issued was AmEx and where I worked it was almost unusable. Fortunately, that was changed to a Diners Club branded Mastercard, and that was widely accepted and had some great perks like free airport lounge use.
Delta was also the first airline on which I made elite level ff status, and I once loved flying that airline. However, Delta has for many years led the race to the bottom for airline perks, especially ff perks for passengers. They were also the first US airline to take away free alcoholic beverages on trans-oceanic flights. Most of the downgrading of ff programs happened during merger mania, but Delta tried a massive downgrade years before the mergers. A group of Delta passengers on the FlyerTalk site fought back and raised money to challenge them with the Save Sky Miles organization, passing out literature on how Delta was screwing passengers at airports, putting up billboards near Delta hub airports, and even flying a plane with a banner over their stockholders meeting. After about a year, Delta caved and restored all the SkyMiles Medallion benefits. I did not wait, and had already comped my elite status over to Northwest, which I found to be a much better airline, anyway. One obvious difference was that I was getting a heck of a lot more first class upgrades on NW than I ever did on DL, incluing on international flights (except there the upgrades were to business class).