Like my man Marlo said, 'Paid in Full' by Eric B and Rakim is on the top of any greatest of all time list
My other picks:
Like your first girlfriend, you do not forget your first rap. And that was 'Rapper's Delight'. I could do that by heart back then, was blowing my mind.
Then, I LOVED Ice-T with his Body Count album. Tha same-named track still gives me goosebumps
'New York, Ney York' by Grandmaster Flash and The Furious Five.
Let's not forget the ladies. 'Let's talk about sex' by Salt'n Peppa was breaking open a lot of doors for so many women in the music biz and did a lot for the topic of sex.
Last, but far from least: Missy Elliott - Work It
She is so original, she has it all.