Since taking over Berkshire Hathaway in 1965, Warren Buffett has delivered phenomenal returns for investors. For example, if you invested $100 in Berkshire Hathaway in 1965, it would be worth more than $2.4 million today.

Buffett's secret to success is buying shares of high-quality companies and holding on for years, even decades. Two intriguing Buffett stocks to buy hand over fist in March are American Express (AXP -1.58%) and Markel (MKL -0.01%).

1. American Express

American Express provides credit cards to customers and processes payments globally. It sets itself apart from the competition with its premium customer base drawn to its high-end card products. American Express wants customers to see its cards as a sign of luxury. It recently revamped its Platinum card product, which carries a $695 annual fee, by adding thousands of bonus points for customers and other nontravel perks to draw in new customers.

Its strong brand recognition is what drew Buffett to the company as a long-term investment. In an interview with Bloomberg, Buffett said, "You can't create another American Express. I could create another shoe store, I could create another business publication. I could do all kinds of things with hundreds of billions of dollars, but I can't put in the minds of people what is in their minds about American Express." 

A person in a retail store pays using a credit card.

Image source: Getty Images.

American Express makes most of its money on discount revenue, or the fees merchants pay when they accept payments with its cards. Last year, discount revenue accounted for 58% of its total revenue. It also earns interest income from loans outstanding and customer fees, such as those annual fees paid on its premium products.

Last year its revenue jumped 22%, outpacing competitors Visa and Mastercard, whose sales grew 18.5% and 17.8% in comparison, respectively. It did a stellar job of gaining younger customers, adding more than 12.5 million new cards, with millennials and Gen Z representing more than 60% of these new acquisitions. 

A slowdown in the economy could affect American Express's customers, who may have to cut back on spending. However, its high-end customer base could maintain spending despite a recession or persistent inflation.

American Express is optimistic about this year, and it expects revenue to rise by 16% and earnings to increase by about 14%. American Express's strong brand and robust customer base keep growing, making this an excellent Buffett stock to buy hand over fist and hold for the long haul.

2. Markel

Markel is a specialty insurance company writing policies covering niche markets, including liability for highly specialized business professionals, wind and earthquake protection, equine insurance, and other environmental insurance.

Markel relies on its deep expertise and specialized market knowledge to sell policies to customers that standard insurers won't touch. As a result, it can write policies that are more expensive or restrictive than standard insurance, resulting in higher profit margins.

Buffett is a longtime fan of insurance companies, cites them as a key component of Berkshire's long-term success. Buffett likes that insurance products "will never be obsolete, and sales volume will generally increase along with both economic growth and inflation." 

He also likes the cash flows these businesses generate. Insurers are great for cash flow because they collect premiums upfront and can invest that cash to generate additional revenue. As long as an insurer keeps writing profitable policies, its investment portfolio can continue to grow.

MKL Free Cash Flow Chart.

Data source: YCharts.

Among other things, Markel is well-known for its large diversified investment portfolio, topping $27 billion. While insurers tend to invest in mostly fixed-income assets like U.S. Treasuries, Markel invests nearly one-third of its portfolio in stocks -- earning it the nickname "Baby Berkshire." Over the past 10 years, the equity portion of its portfolio has delivered annual returns north of 13%. 

Markel does a solid job of writing insurance policies and an even better job of managing its investments. Conditions look favorable for insurers such as Markel, and its investment portfolio has it well positioned for the next bull market in stocks.