This article is from the Car Audio FAQ, by Ian D. Bjorhovde (firstname.lastname@example.org) with numerous contributions by others.
The single most important attribute is the capacitance value (expressed
in Farads). Put simply, more is better. Another important
consideration is to make sure the maximum voltage rating of the
capacitor safely exceeds the operating voltage of your vehicle's
electrical system. In addition, ESR and ESL values may be provided
with some capacitors to essentially indicate the amount of voltage drop
that occurs when a capacitor is delivering current. Smaller values are
better in this regard.