Greatest is influenced by factors other than technical skill.
Jay-Z I would say is a safe bet for greatest when you add in his accomplishments...but by no means is Jay a clear cut better MC than someone like Rakim or Nas or to be real, even someone like Black Thought.
But none of those 3 have been able to generate the amount of success that Jay has.
Pac is a great MC and is very much a legend (that post somewhat inspired this one), but is he a better MC than, let's say Monch?