Exchange-traded funds have been raking in the dollars in recent years -- to the point where they now carry more than $1 trillion in assets. With their low costs, greater tax efficiency, and simple match-the-market approach, it's easy to see why ETFs are a favorite with investors. But, as with actively managed mutual funds, there are a number of ETFs out there that aren't appropriate for the average investor. Let's take a look at the 25 largest ETFs as measured by net assets and see which of these big funds you might be better off avoiding.

Three to avoid
Clocking in at No. 14 on the list of biggest ETFs with $13.1 billion in assets is iShares MSCI Brazil Index (NYSE: EWZ). This firecracker of a fund has posted a 10-year annualized return of 20.6%, more than 15 percentage points ahead of the MSCI EAFE Index. While it may seem counterintuitive to argue with such incredible trailing returns, this fund is full of risk. As might be expected, wide swings are the order of the day here, with the fund falling 54% in 2008 and then rebounding 121% the following year. No doubt that performance has been good, but this comes with a huge emerging markets bull market at the fund's back. I generally advise folks to stay away from single-country funds like this. Rather, get your emerging markets exposure from a more diversified option that doesn't hang its hat entirely on the fate of one nation's market.

With $10.4 billion in net assets is the Energy Select Sector SPDR (NYSE: XLE) at No. 19 on our list. Given the rise in oil prices we've seen in recent months, it's not too surprising to see this fund up 18.3% so far this year. And while energy prices are likely to remain high for the immediate future, investors need to be careful with sector-focused funds. When they have a good run, sector funds can post outsized returns far in excess of the broad market, attracting a lot of investor attention. But these funds can just as quickly turn cold and post large losses. It's very difficult to accurately time peaks and troughs within sectors, so a better bet is to stick to diversified, wide-market funds instead.

Lastly, the iShares FTSE China 25 Index Fund (NYSE: FXI) lands at No. 25 on our list with $8.2 billion in assets. And while few would argue with the long-term potential of the Chinese market, again, single-country funds are far riskier than their more diversified global counterparts -- not to mention that country funds like this are usually much more expensive than broader market ETFs. This China fund will set you back 0.72% a year; that's much less than a similar active fund would cost, but it's pretty pricey for an exchange-traded fund. And since the fund tracks only 25 of China's biggest companies, you're not getting wide coverage of the Chinese market. That just adds to the risk levels already present. Again, investors would be better off with a more diversified emerging markets fund.

And a handful that you shouldn't miss
Fortunately, investors are probably getting a lot right, since most of the biggest ETFs around are solid, well-diversified options. Topping the list of large-and-in-charge ETFs is the SPDR S&P 500 ETF (NYSE: SPY) with more than $95 billion in net assets. Likewise, Vanguard Total Stock Market ETF (NYSE: VTI) lands in the No. 9 slot, with $20.2 billion to its name. Both funds are excellent options for gaining wide exposure to the domestic stock market and come with rock-bottom price tags of 0.09% and 0.07%, respectively.

Similarly, the single best emerging markets ETF offering also makes an appearance in the top 25. Vanguard MSCI Emerging Markets ETF (NYSE: VWO) is now the third-largest ETF in existence with $49.3 billion in assets. For just 0.22%, investors can get broad exposure to developing nations across the globe. Fixed-income investors of all stripes would also do well with the No. 21-ranked ETF, Vanguard Total Bond Market ETF (NYSE: BND). This fund tracks a wide swath of the domestic bond market, including Treasuries, mortgage, and corporate bonds, so bond-lovers can get all their bases covered in one stop here.

And two to use with caution
Perhaps not surprisingly, the No. 2 spot on our list of largest exchange-traded funds goes to SPDR Gold Shares (NYSE: GLD), while iShares Silver Trust (NYSE: SLV) lands at No. 11. These funds boast $60.6 billion and $17.3 billion in net assets, respectively. While performance in the precious metals sector has been off-the-charts in past years, investors need to exercise caution when buying into inflated markets like these. These two funds are actually pretty decent options for cheap exposure to gold or silver. So if you're looking for a small, long-term metals hedge, either fund would probably work for you. But investors who are piling into gold and silver based on recent returns are facing a good possibility of getting burned. Prices are fairly high here, and given the stellar run of metals like these, I think a correction is more likely than another 10-year run of exponential returns.

Remember, just because an exchange-traded fund has a big asset base and is attracting a lot of investor inflows doesn't mean it's the right fund for you. Make sure you stick with low-cost, broad-market, well-diversified funds and leave the riskier offerings to the speculators.

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