My preferred rock is a fast coti, though the combined result of my skills and my stone only get the keenness to a low-mid 8k level of keenness. Not all blades respond evenly (or I don't work them evenly), and if I have to fuss too much, I reach for the Nani 8k.

If cotis were as consistent as the synthetics - or you have either the funds to buy and sell a bunch to find the golden rock, or the fortune to get one you instantly work well with - they would be hard to beat.