I know I'm resurrecting this thread from the depths, but I can't believe there wasn't a single mention of arguably the greatest album in hip hop history:
"Paid in Full" - Eric B & Rakim
Most everyone in the industry of note from hip hop's "Golden Age" will cite this album as the inflection point that expanded their perception of the art of rhyming as well as production. And if you think about who preceded Eric B & Rakim, it's easy to see that "Paid in Full" was the jumpoff point for styles and rhythms to evolve.
To use a football analogy, they were the Bill Walsh of hip hop & rap, spawning many greats who took the genre into new spaces.