Question: what is the best stock index to invest in?
Answer: Fidelity ZERO Large Cap Index (FNILX) The Fidelity ZERO Large Cap Index mutual fund is part of the investment company's foray into mutual funds with no expense ratio, thus its ZERO moniker. ... 
Vanguard S&P 500 ETF (VOO) ... 
SPDR S&P 500 ETF Trust (SPY) ... 
iShares Core S&P 500 ETF (IVV) ... 
Schwab S&P 500 Index Fund (SWPPX)