Depends on what you consider the best. You want to shoot like a laser or cover a barn door? Either way, there are variances between 2 guns using the exact same choke.
Pick a brand of choke you like and try something in the .555 to .570 range for a 20 gauge. If you get lucky, the first one will be perfect. If not, try another size.
I shoot Boss 2oz 9s through a .555 in a remington 870 and .555 and .565 in tristar O/U. The .555 in both guns is a very small pattern so up close requires a tight aim.