SPY is the best-recognized and oldest US listed ETF and typically tops rankings for largest AUM and greatest trading volume.
SPX, the index is widely used as a barometer for the health of the U.S. stock market, representing roughly 80% of US stock market total value.
BlackRock iShares XSP is an exchange-traded fund designed to replicate the performance of the S&P 500 (Hedged to Canadian dollars).