The Kaiser-Bessel window is a flexible window whose shape you can modify by adjusting the parameter beta. Thus, depending on your application, you can change the shape of the window to control the amount of spectral leakage. Kaiser-Bessel windows for different values of beta are shown in the following illustration.
Note that for small values of beta, the shape is close to that of a rectangular window. Actually, for beta = 0.0, you do get a rectangular window. As you increase beta, the window tapers off more to the sides.
This window is good for detecting two signals of almost the same frequency but significantly different amplitudes.