The best risk/reward in $SPX occurs when VIX is under 20. Average monthly returns (1.5%) and the maximum downside (-4.4%) occur during these periods. On average, monthly returns are positive 77% of the time.
For comparison, when VIX is 25-30, average monthly returns drop (0.2%), maximum downside doubles (-9.1%) and the monthly returns are positive only 52% of the time.
It gets worse the higher the VIX. A nice report can be found here with a the summary table extracted below.