It’s probably a good sign when one of the greatest investors of all time holds a big stake in a stock that we’re considering for a portfolio addition.One of Warren Buffett’s long-held positions, American Express just delivered its fourth consecutive earnings beat. A Zacks Rank #2 (Buy) stock, American Express continues to defy its skeptics.A global provider of credit card payment products and travel-related services, the company operates through four segments: US Consumer Services, Commercial Services, Inte ...