Is VOO Still the Best S&P 500 ETF You Can Own? Here's How It Stacks Up Against the Alternatives.

The bigger, the better? Some investors could think so when it comes to S&P 500 (^GSPC +0.66%) index funds.

The Vanguard S&P 500 ETF (VOO +0.68%) ranks as the largest exchange-traded fund (ETF) by assets under management (AUM). This low-cost ETF has earned its popularity with investors through the years. But is it still the best S&P 500 ETF you can own?

Two other S&P 500 funds are breathing down the Vanguard S&P 500 ETF's neck in the AUM ranking -- the iShares Core S&P 500 ETF (IVV +0.67%) and the State Street SPDR S&P 500 ETF (SPY +0.67%). The State Street SPDR Portfolio S&P 500 ETF (SPYM +0.68%) isn't too far behind. Each of these ETFs is a strong contender and gives Vanguard's fund a real run for its money.

These funds share much in common. They all seek to track the performance of the S&P 500 index and own the same large-cap stocks. Their returns are, unsurprisingly, nearly identical.

One differentiating factor, though, is cost. Vanguard is known for its low fees. The Vanguard S&P 500 ETF lives up to this reputation with an annual expense ratio of 0.03%. That's well below the annual expense ratio of 0.0945% for the State Street SPDR S&P 500 ETF.

However, Vanguard's popular S&P 500 ETF has stiff competition on this front. The iShares Core S&P 500 ETF also has an annual expense ratio of 0.03%. And the State Street SPDR Portfolio S&P 500 ETF narrowly beats both of these funds with an expense ratio of only 0.02%.

If you're looking to buy and sell options, the State Street SPDR S&P 500 ETF is arguably the best bet. Its trading volume is much higher than that of other S&P 500 ETFs. As a result, the fund's bid-ask spreads can be more attractive.

Long-term investors should be able to win with any of these four S&P 500 ETFs. Holding the State Street SPDR S&P 500 ETF instead of one of the other three probably isn't the best strategy, though, unless you're trading options regularly due to the higher cost.

The Vanguard S&P 500 ETF, the iShares Core S&P 500 ETF, and the State Street SPDR Portfolio S&P 500 ETF charge such low fees that cost is almost irrelevant. However, for anyone seeking the best S&P ETF to buy, there's a pretty good argument to be made that the State Street SPDR Portfolio S&P 500 ETF is actually the best pick today because of its super-low expense ratio.