Is there a best or top-tier capacitor for a Zobel network?

It doesnt matter. The Zobel is a shunt. Even a $1 bipolar electrolytic will do. The whole reason for the Zobel is to induce high frequency LOSS for the woofer. The lossier the cap is the better. Save money for the high pass cap for the midrange and tweeter.

From Rich Brkich of Sif=gnature Sound:

"Likely different resistors, capacitors and wire will change thesound of the RC to some degree - as will changing the valueswhich will effect to frequency in  which RC starts being effective.

I can't tell you how because besides the difference betweenthe Standard RC and the Master RC that I have direct experience with - I have not tried making and listing to RCs with all these various parts or different values."