Benchmark, is discussed a lot in the community. According to my opinion, SPY is good choice, but Quantopian should also provide some options by the users. Say I have a strategy of trading AAPL, then I should be available compare my strategy return with Apple stock return, but not with SPY.
And if I have a strategy of two or three equities, may be I want to see each of them as benchmark.
Last, if I have trading strategy of 30 equities, showing all of them will be messy, and thus I hope that the benchmark is the average of these 30 stocks return, but may be user defined weight feature will be better.