What is the most useful financial statement for an investor ?

It depends on the investor. Income statements shows  how the company can generate its profit, and its easier to analyze.

Balance sheet: gives a detailed information about the assets, liability and equity, with useful ratios such as profitability ratios. works better when paired with the income statement.

Cash flow statement: gives an overview about the cash inflow and outflow based on operation, investing and financing activities, giving the investor an idea where the company spend its money on.