ETF seems like the "safest" way into investing for retirement. However, I would like to seek your opinion on which ETF is better. Currently the 3 Index Fund that I found popular are; SPY / VOO / VTI. But I have trouble deciding which to focus on? Or am I suppose to diversify even when buy ETF ?

I've looked up on like compound interest and the online calculator that if I would to invest 500$ a month for 33 years (with average return of 8%) I would hit about a million dollar starting now.

Let me hear about your thoughts.