The answer to this question depends on how you plan to use the money you're investing. If you're saving for retirement, paying off debt, or making a large purchase in the near future, cash is your best bet. For long-term equity growth or income, stocks are likely your best option.
Cash is always a safe investment in any market condition. It provides liquidity, which means you'll always be able to access your capital with no penalty when you need it most. If you're saving for something big in the near future, cash will provide the flexibility needed to take advantage of opportunities that may arise.
Cash has historically outperformed inflation, so while your investment won't grow in value, it will maintain its purchasing power. This is generally the best option for emergency funds or other short-term savings.
Stocks provide the best long-term growth and tend to outperform cash in any market condition. While they're riskier than cash, the better potential growth rate makes them a good option for long term investing. Even if you hold your stocks for just a few months at a time, you'll enjoy the long-term benefits of equity growth.