I want to add that the Bessel filter is a linear form of filter that provides a maximally flat group delay or propagation delay across the frequency spectrum, but offers a slower transition from pass-band to stop-band than for other forms of filter of the same order.
However, the Bessel filter is not used as widely as the Butterworth of Chebyshev filters, although the fact that it has a maximally flat group delay means that Bessel filters are often used in audio applications such as audio cross-over networks.
http://www.radio-electronics.com/info/rf-technology-design/rf-filters/bessel-rf-filter-basics.php