Warren Buffett is stepping down. Here's the one stock he's betting 28% of his portfolio on.

Warren Buffett, one of the most renowned names in the world of finance, was inspired by two famous economists from the 1930s, Ben Graham and Philip Fisher.

Amid the current economic uncertainty, American Express remains strong thanks to its cardholder base, so it makes a great stock to own during good times and bad. And today, trading at only 15 times forward earnings estimates, it's a fantastic bargain for long-term investors.