Joseph,
I did not mean that nobody can judge among gedolim, but rather when dealing with the very great, for most people, most of the time, it is impossible to tell who is greater. If I spent years with the Chofetz Chaim and then years with Rabbi Aryeh Levine, then even I might be able to make a judgment on who is the bigger tzaddik, but practically speaking that isn’t possible.
There is also a difference between quantifiable statements of practical worldy impact and qualitative statements of people’s greatness. We can say that Rav Ovadiah was the leading Sefardi posek of his times because practically speaking his piskei halacha were more wildely followed than anyone elses were. Similarly, we can that Reb Moshe was the leading posek in America in the 20th Century. But that doesn’t mean that Reb Moshe is qualitatively greater posek than Reb Yaakov Kaminetsky or Rav Henkin and certainly not that he greater overall. I am not qualified to judge between Reb Yaakov, Reb Moshe, and Rav Henkin. There are those that might be qualified, but they will be very few and far between.
With regard to comparing Acharonim with Rishonim and Rishonim with Tannaim or Amoraim: it isn’t that I have a way of personally being able to judge between the Rambam and Rabbi Akiva. It is that the Rambam himself, and all other Rishonim, did not believe themselves to be capable of arguing with Rabbi Akiva (unless they had another Tanna to support them).