Warren Buffett, George Soros, and Carl Icahn are famous investors worth billions of dollars, but these three gurus embrace wildly different investment approaches. Buffett buys great stocks at fair prices for the long haul; Soros invests opportunistically to benefit from both bull and bear markets; and Icahn is an activist investor who buys big and then advocates from within for shareholder-friendly change. These money managers have proven that all three approaches can be savvy ways to invest, so let's take a look at their recent buys.

Unlocking pent-up demand

Warren Buffett's biggest buy recently has been Apple Inc. (NASDAQ:AAPL), the consumer-electronics giant.

IMAGE SOURCE: THE MOTLEY FOOL.

Apple first showed up in Buffett's Berkshire Hathaway (NYSE:BRK-B) (NYSE:BRK-A)portfolio in the first quarter of 2016, when shares were in the doldrums because of worry over slowing demand for iPhones, due to consumers holding off on upgrades.

While iPhone sales growth was declining, Apple's business overall was still humming along (and kicking off tremendous cash flow in the process). Apple's strong brand still allowed it to charge profit-friendly premium prices, and revenue from services, such as iTunes, was still climbing; that was enough to convince Buffett it was worth picking up this company's shares from the bargain bin.

Today, Apple is Berkshire Hathaway's third-largest position behind Kraft Heinz and Wells Fargo, and with shares up 60% in the past year, Buffett's got to be smiling about his profit so far.

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AAPL 1 YEAR PRICE RETURNS (DAILY) DATA BY YCHARTS.

Recently, Buffett suggested he's finding it harder to justify buying more shares in Apple now that it's run so much higher, but he doesn't seem to be in a hurry to book his gain, either.

Instead, he appears willing to roll the dice a bit longer to see whether or not the fast-approaching iPhone 8 will be feature-rich enough to compel people to finally upgrade their old iPhones. Only time will tell what happens this fall, but rumors of OLED displays and 3D sensing could be enough of an improvement to create a tidal wave of unit volume. If so, Apple's shares could still head higher.

Defense or offense?

Drugmakers are defensive stocks because demand for medicine is less prone to the whims and whispers of the economic cycle. Perhaps that's why George Soros started a position in pharmaceutical Goliath Pfizer Inc. (NYSE:PFE) last quarter. Or maybe Soros simply likes Pfizer's potential to be a leader in the emerging multibillion-dollar biosimilars market.

Biologic drugs are made in living organisms and thus are nearly impossible to copy exactly. However, despite being inexact copies, biosimilars provide similar efficacy and safety.

Over the past decade, complex biologics have become some of the world's best-selling medicines. However, many of these biologics, including Johnson & Johnson's Remicade and Amgen's Neupogen, have lost or will soon lose their patent protection. To capitalize on that opportunity, in 2015, Pfizer forked out $17 billion to acquire biosimilar-drug developer Hospira.

Late last year, Pfizer won approval from the U.S. Food and Drug Administration for Hospira's Remicade biosimilar, Inflectra; recently, a key FDA committee recommended approval of its biosimilar to the blockbuster anemia drug Epogen. In Q1, Inflectra sales helped Pfizer's biosimilar revenue grow 62% year over year. And while it will be a while before we realize biosimilars' peak sales potential, tens of billions of dollars in biologic sales are up for grabs in coming years as drugs like the $14-billion-per-year Humira lose patent exclusivity.

Granted, George Soros is typically in and out of stocks relatively quickly, so he probably won't stick around in Pfizer long enough to realize biosimilars' potential. Nevertheless, his interest in the company may suggest to investors that it's a good time to give Pfizer a close look.