This site says that a good cap rate for a rental property is between 4 and 10 percent.
The S&P's average annual return for the past ninety years is 9.8 percent.
This begs the question: Why would you invest in residential real estate rental property if a "good" rate of return on your investment is no better than what you could make on average in the stock market? The latter income is more passive (you don't have property to manage) and your assets are more liquid (you can sell your stocks at any time).
The only reason I can think of is that the stock market is probably more volatile than real estate. Are there are other reasons why real estate could be a better investment than stocks, particularly if you are focused on long-term average returns?