Here’s a breakdown of some key investments Warren Buffett made through Berkshire Hathaway and their estimated market capitalizations at the time he started investing:

1. **GEICO (Insurance) – 1976:**
   - **Market Cap at the time:** Around $50 million.
   - Buffett had first invested in GEICO personally in the 1950s. However, it wasn’t until 1976 that Berkshire Hathaway began purchasing a significant stake in the company when it was struggling. In 1996, Berkshire fully acquired GEICO.

2. **Coca-Cola (Consumer Goods) – 1988:**
   - **Market Cap at the time:** Around $18 billion.
   - Berkshire began purchasing shares of Coca-Cola in 1988 following the stock market crash of 1987. Over the years, Berkshire accumulated a large stake, and this investment has grown to be one of the company’s most profitable.

3. **American Express (Financial Services) – 1964:**
   - **Market Cap at the time:** Around $150 million.
   - Buffett started buying American Express after the "salad oil scandal" in the mid-1960s, when the company's stock was significantly undervalued due to a major financial crisis. Berkshire Hathaway now holds a sizable stake in American Express.

4. **Washington Post (Media) – 1973:**
   - **Market Cap at the time:** Less than $100 million.
   - Buffett began buying shares of the Washington Post during a period of market turmoil in the early 1970s. The investment was a long-term holding and turned into a very profitable one for Berkshire.

5. **Apple (Technology) – 2016:**
   - **Market Cap at the time:** Around $540 billion.
   - Berkshire Hathaway began buying Apple stock in 2016. This was a major shift for Buffett, as he had typically avoided tech stocks. It has since become one of the largest investments in Berkshire's portfolio and one of its most profitable.

6. **Burlington Northern Santa Fe (BNSF Railroad) – 2009:**
   - **Market Cap at the time:** Around $34 billion.
   - Buffett began buying BNSF in 2009 and acquired the entire company for around $44 billion in 2010. This was one of Berkshire's largest acquisitions at the time.

Each of these investments came at times when Buffett believed the market was undervaluing these businesses. His strategy has always been to buy companies with strong fundamentals during periods of uncertainty or market distress, where they had significant growth potential or a strong competitive advantage.