Exchange-traded funds have made it easier than ever for investors to build a diversified portfolio. But even though there are well over a thousand ETFs to choose from, you don't need to own a huge assortment of ETFs to establish a solid set of core positions for your investing strategy.

In fact, looking at the top ETFs by assets under management according to the ETF Database, you could make a good argument that just picking from the biggest ETFs in the market would leave you with a reasonable starting point for a diverse set of investments. Let's look at four of the top five ETFs to see whether they belong among your holdings.

SPDR S&P 500 (SPY 0.14%)
With more than $135 billion in assets under management, the original Spider was the first ETF to become a household name among investors. Tracking the S&P 500, the SPDR ETF has rock-bottom expenses of just 0.09% annually, and its heavy trading volume and relatively high share price make the frictional costs of buying and selling shares as small as possible.

Nearly all U.S. investors like to have core exposure to large-cap U.S. stocks, and the S&P 500 is one of the most popular benchmarks that both active and index investors use to evaluate their overall performance. An equivalent iShares ETF that also tracks the S&P 500 makes the top five list, but you don't need more than one S&P-tracking ETF in your portfolio.

Vanguard Emerging Markets (VWO 0.72%)
Emerging markets have fallen out of favor recently, with some investors worrying that the Federal Reserve's coming pullback on the extent of its quantitative easing program will have a negative impact on growth in emerging economies. The argument is that cheap financing has made it easier for U.S. investors to get money to invest in emerging markets, and so when rates start to rise, emerging markets will be the first to feel the impact.

Nevertheless, the Vanguard ETF has bounced back more recently as value investors have argued that shares have fallen far enough to make emerging markets a bargain. With annual expenses of just 0.18% and assets approaching $50 billion, the Vanguard ETF charges much less than many international funds, and exposure to companies in countries like China, Brazil, and South Africa could help your portfolio grow even when the U.S. isn't performing well.

iShares MSCI EAFE (EFA 0.34%)
Emerging markets aren't the only place to invest internationally. Developed markets in Japan and across Europe have also seen their ups and downs lately. But Japan's efforts to devalue its currency to spur export growth has led to new interest in its stock market, and despite a recession in Europe, investors are starting to gain confidence in turnaround efforts there.

With fees of 0.34% annually, the iShares ETF isn't the cheapest way to play the international market. But with $44 billion under management, many institutional and individual investors alike rely on the fund for their core international exposure outside the emerging world.

SPDR Gold (GLD 1.65%)
Gold has performed abominably lately, making the gold-tracking SPDR Gold ETF much less popular than it once was. For a while, the gold ETF challenged the SPDR S&P 500 for supremacy as the ETF with the most assets under management. Lately, gold's price plunge has led to many former shareholders having exited the fund, imposing a double-hit on its asset levels. Yet the ETF is still worth about $40 billion, with about 29.5 million ounces of gold bullion in its coffers.

With expenses of 0.4% annually, SPDR Gold charges more than some smaller counterparts. Moreover, the shares are treated as collectibles for capital-gains tax purposes, leading to higher maximum rates applying to many shareholders. With many investors seeking commodity investments to hedge their stock exposure, SPDR Gold can serve a useful role for many portfolios.

Be ETF-smart
Obviously, these four ETFs don't cover every investment you could be interested in, with small- and mid-cap stocks and fixed-income securities being obvious holes. But the huge amount of assets these ETFs have accumulated are a testament to their usefulness for many investors, and they're a reasonable place for you to start as you seek to build an ETF portfolio of your own.