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Millions of investors have trillions of dollars invested in mutual funds, and many top funds performed roughly in line with the broader stock market's returns in 2015. That left many fund investors with relatively lackluster returns given the flat performance for major market benchmarks last year, but a few funds stood out for their outpaced gains. Below, you'll find three funds with assets of $10 billion or greater that posted some of the best returns in 2015.

Fidelity Select Biotechnology
The biotechnology industry had a strong 2015, and many investors who picked the right stocks in the biotech sector were rewarded for their willingness to take risk. The Fidelity Select Biotechnology Fund (FBIOX), which boasts about $15 billion in assets under management, gave investors a solid performance in 2015, posting returns of about 14% on the year.

Among the top contributors to performance were stocks like Regeneron Pharmaceuticals (NASDAQ:REGN), which jumped more than 30% on the year. The fund also had positions in a couple of small companies that more than doubled in 2015, including Genmab and Anacor Pharmaceuticals. On the flip side, the fund managed to avoid some of the larger losers on the year, at least among its top holdings. If biotech can put together another good year in 2016, then the Fidelity fund could once again appear among the best funds in the mutual fund universe.

Harbor Capital Appreciation
Among broader-based funds, the Harbor Capital Appreciation Fund (HACAX) weighed in with gains of 11%. The $27 billion fund was fortunate enough to have exposure to a couple of important trends while avoiding the pitfalls that held back many of the fund's counterparts.

First, the Harbor fund took full advantage of the boom in large technology companies, with a No. 2 holding in Amazon.com (NASDAQ:AMZN) and a top-10 position in Netflix (NASDAQ:NFLX), both of which more than doubled on the year. The fund also held strong-performing consumer stocks, but it largely avoided energy and materials stocks and their poor returns during the year. With no apparent relief in sight for commodities, Harbor's focus on growth could be a winning move for 2016 as well.

T. Rowe Price Blue Chip Growth
Another solid winner in the broad-market category was T. Rowe Price Blue Chip Growth (TRBCX), a $31 billion fund with an emphasis on quality growth. Like Harbor Capital Appreciation, the T. Rowe Price fund benefited from large positions in Amazon, Netflix, and other high-performing technology stocks.

At the same time, the fund also had solid exposure to good performers in the financial and healthcare arenas, choosing some of the best large-cap stocks in each space. The fund didn't avoid every possible pitfall, with a sizable position in beaten-down Valeant Pharmaceuticals (NYSE:VRX), but overall, T. Rowe Price Blue Chip Growth managed to give investors the benefit of the growth portion of the U.S. stock market without suffering losses from the industrial side of the economy. If that dichotomy continues this year, then the T. Rowe Price fund should be well-poised to capitalize on high-return potential in 2016 as well.

Thinking ahead
These three fund giants all benefited from some of the same factors that helped push portions of the stock market higher in 2015. Coming into 2016, some investors will inevitably think that these funds are due to revert back toward the mean, but many of the economic and financial influences that helped push the stocks these funds own higher look like they'll continue throughout the year. Investors can't reasonably expect Amazon and Netflix to double again in 2016, but they're still building up their respective audiences and will see their fundamental financial health improve over the coming year. Biotechs like Regeneron still have plenty of potential, and even the controversy that Valeant has gone through lately could subside and let the company return to its past success. That makes these funds worth a closer look for investors in 2016 and beyond.

Dan Caplinger has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Amazon.com, Netflix, and Valeant Pharmaceuticals. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.