Hi Lisa, I don't whether I would say it's definitely "best". That is a hard thing to say, but the general rationale is that the Hanning, or Hann window is a good compromise between main lobe width, which determines frequency resolution, and side lobe level, which governs spectral leakage.
As you reduce the side lobe levels, you inevitably worsen frequency resolution by increasing the width of the main lobe.
Conversely, as you narrow the main lobe, you inevitably raise the side lobe level.
The Hann window is in a good middle ground on both measures among the commonly-used windows.