It's a testimony to how priorities in the genre have changed that hardly anyone is compiling lists of the "greatest detectives" anymore - though Sherlock Holmes is routinely assumed to be the greatest of them all. Since most modern sleuths are more remarkable for their personalities and predicaments than their ratiocinative skills, the exercise would be pointless.
Besides, evaluating the "greatness" of a detective is way trickier than seems firsthand for lack of an universal system of measurement. Every fictional sleuth is the greatest one in his own universe, being ideally and uniquely suited to solve the problems he is submitted. Ellery Queen's cases are not the same as Dr. Fell which on the other hand are different from, say, Philip Marlowe's and we don't know whether they could switch successfully. Neither can we be sure that Sherlock Holmes could solve the murder of Roger Ackroyd, or Hercule Poirot would escape from Cell 13. Only by having them all existing in a same universe and investigating the same case could we eventually work out who's best.
So in the end when making such lists we're not listing "greatest" detectives so much as those whose exploits most impressed us - and it might then be more helpful to specify which particular "performance" owes them their ranking.
Using this system, who would be your ten favorite detectives?