It varies a lot between different guns.  Some use a Remington Super Full, some get good results with a Carlson 575, others use a Sumtoy 562, and a a lot of folks use Indian Creek either a 562 or 555.  I have a youth model 870 and finally settled on the IC 555 and I have shot several different chokes.  Shorter barrels may need a tighter choke...although that doesn't hold true all the time.