“The greatest” is a tough term to define.
Level of yiras shomayim (a mentsch) is something only G-d can determine. We can all tell stories about various people but some gedolim are so good at hiding their tzidkus we don’t even know stories about them.
Level of shiur is also a toughie as
A. there are very few people who go to every shiur to really compare
B. does anyone know how to rate a level of a shiur? If one person quotes 10 rishonim and achronim an hour and another quotes 15, does that make it a higher level shiur?
I think a better gauge should be the influence they have on the American Orthodox community.
I would certainly add R. Aharon Lopiansky shlita and R. Hershel Schachter shlita to the top of the list.