Who is the best descender in the world is a question that probably can not be answered. For starters, before the downhill you need to climb, and since the TV footage concentrates on the head of the race, there is a number of riders that we never see going downhill. Cavendish might be the best of them all, for all we know. Boonen has great bike handling, is not afraid of the speed and taking risks (as we can see when he competes regularly in bunch sprints), and is one of the heaviest pros around. He has all the components to make a great descender yet he is not even in the conversation. Why ? Because we never see him going downhill.
Therefore the only question we might answer is out of the strongest riders of the peloton, which one descends better ? Even then there are a number of problems that can not be solved. For example, a key component of descending well is braking at the last moment but it is not something that we can easily measure in front of our TV especially since the riders of the group have to adjust to the rhythm of the rider leading the group which means that we can not make a direct comparison between two riders.
Sagan closed down a 20 second gap on a fresher Cunego downhill during the last Tour de Suisse, and it is as close to an answer that we can hope to get about the relative descending skills of two riders (one on one during a long and fairly technical descent) so Sagan can reasonably be said to be better than Cunego (who is still very good downhill by the way). Therefore I can not see how Cunego could be the “best”. Same with Samuel Sanchez, he got dropped by Evans on a downhill which in turn could not keep up with Nibali. And since a few years the Spaniard has been pretty unspectacular downhill, therefore I can not see how he could be considered the best descender, let alone be picked by almost twice as much people than Sagan.
As for those that say that Nibali has no technique, well actually try to take your fanboy glasses off and look at his trajectories which are pretty much perfect for 180 degrees turns and even if it can get a little worse when dealing with a rapid succession of not so sharp bends, he probably has one of the best, if not the best descending skills of the entire peloton. Cancellara might still descend faster because he also takes corners well and he is much heavier but since Nibali is basically the only rider that uses his downhill skills consistently to try and win races, that means it is a bigger asset for him than for others (or at least he considers it to be an asset which is not the case of the others riders) and that is why I would give him the slight edge.