I like to learn from the stock picks and styles of the great investors and traders. Along these lines, I like looking at the 13F-HR filings made by institutions and funds with more than $100 million in stock -- filings that are required by the SEC every quarter.At Stockpickr.com, we strive to track the investments of every major hedge fund, mutual fund and mega investor, and I plan to pick an investor once a week and write an article based on their 13F-HR filings to get a sense of their approach and offer a look at their picks. This week, I am highlighting Lone Pine Capital, which was started in 1997 by Steven Mandel, a former managing director and analyst at Tiger Management. In 2007, according to Bloomberg, Lone Pine returned 46.5%, handily crushing the average 11% returned by hedge funds that year. Neither a growth nor value investor, Mandel looks for good companies run by smart people, with stock valuations below what he deems to be fair market value. One of Mandel's largest investments is in data storage company EMC ( EMC). Mandel first initiated a position in EMC right before the successful partial spinoff of VMware ( VMW) in late July. After receiving a series of upgrades back in October from sell-side analysts, EMC shares have fallen from their high of $25 back to the low-$16 range. Despite being down 25% for the past three months compared with the Nasdaq's 15% downturn, EMC is one of the cheapest stocks around. If you discount its 86% stake in VMware, you get a $5.30 stock that has grown revenues 10%-plus since 2002. EMC has $5.8 billion in cash as well as a $2 billion buyback in place. Another one of Mandel's positions is in Nalco Holding ( NLC), a worldwide provider of water treatment and processing services. The stock just traded to a new 52-week low, even though "drinkable" water possibly could be the next big trend to hit Wall Street. Someone once told me that in theory, the world was only meant to hold 1 billion to 2 billion people maximum. Clearly, we are well past that number, and with only 1% of total water on the earth deemed drinkable, we have a problem. In fact, many geologists forecast a water shortage within the next 10 to 15 years. For Nalco and its shareholders, this forecast could not be better. The company's industrial and institutional services segment offers water treatment applications to prevent corrosion, contamination and the buildup of harmful deposits in large sources of water worldwide. With a forward price-to-earnings (P/E) ratio of 15, a P/E-to-growth (PEG) ratio of 1.3 and operating margins well above the industry average, Nalco is a great investment. Another one of Mandel's positions is in retailer Dick's Sporting Goods ( DKS), which has taken a drubbing since the fall despite an incredible earnings report back in November. Right now the market is selling off any company that has retail exposure -- the classic "sell first, ask questions later" phenomenon appears to be taking place. Back in November, Dick's reported monster third-quarter earnings growth of 57% and sales growth of 18% year over year. Management also raised fourth-quarter guidance. Earlier this week, management upped its outlook for a second time. This boost assures investors that the quarter is going to be good, despite all of the negativity in the stock.