SPY is the best-recognized and oldest US listed ETF and typically tops rankings for largest AUM and greatest trading volume. The fund tracks the massively popular US index, the S&P 500. Few realize that S&P's index committee chooses 500 securities to represent the US large-cap space—not necessarily the 500 largest by market cap, which can lead to some omissions of single names. Still, the index offers outstanding exposure to the US large-cap space. It’s important to note, SPY is a unit investment trust, an older but entirely viable structure. As a UIT, SPY must fully replicate its index (it probably would anyway) and forgo the small risk and reward of securities lending. It also can’t reinvest portfolio dividends between distributions; the resulting cash drag will slightly hurt performance in up markets and help in downtrends. SPY is a favored vanilla trading vehicle.