Question

Hi,
Is it a mistake to rank the best people in a certain field based on specific achievements of theirs?
For example:
Grigori Perelman (Jewish, by the way), who proved the Poincaré conjecture, would be considered the best mathematician of our time, because he is the only one who solved one of the Millennium Problems.
Immanuel Kant would be considered the greatest philosopher of all time, simply because he was just that good.
Is there really such a thing? How can you rank people at all based on a specific achievement? Maybe if Perelman hadn’t proved the conjecture, someone else would have come along and proved it.