best app for american express for my ipad air

The Official Amex App would be the one recommended. https://apps.apple.com/us/app/amex/id362348516#?platform=ipad

I doubt there would be any other that can access your card information. Nor would you want o just let a 3rd party have access to that.

Get the official AMEX app in the Apple Store app.

Info: https://www.americanexpress.com/en-us/support/digital/amex-mobile-app/