Question
Who was the last great person (on a Moses-like scale) to unite the jewish people?
Answer
Unfortunately, I do not think that your presumption that Moshe (Moses) united the Jewish people is necessarily true. Over and over we read in the Torah of splinter groups, or rebelling individuals.
However, if your intention is that Moshe was a leader over the entire Jewish nation, then Rechovoam son of Solomon was the last king to rule over a united Israel.
Yochai Robkin