Warren Buffett’s greatest achievement is not beating the market — it’s his disciplined investing process that all investors can learn from. - MarketWatch photo illustration/Getty Images, iStockphoto As Warren Buffett exits his CEO role at Berkshire Hathaway BRK.A BRK.B, many will revisit his investing milestones — including his purchases of stakes in American Express AXP, See’s Candies, Coca-Cola KO and Apple AAPL — and his prudent pearls of investor wisdom: margin of safety, circle of competence, look-th ...