Well, there are at least two presuppositions to a post like this:

1. That individuals are capable of unique achievements separate from their context, trends, etc.

2. That doing some intellectually impressive thing is "great", in a values or ethics sense. There are many things listed here that other intellectuals have argued as having extremely negative consequences for human society, culture, etc.

Which is why I think a list of the "greatest" is inherently a bit flawed, and you're better off looking at a list of "influential" people or ideas instead.