wow. Great question.
I think mine was "Basketball" by Kurtis Blow. That's the first one I can remember ... I know I saw it on the half-hour video show "Nick Rocks" on nickelodeon. Had to be 1984.
Here's the weird thing -- I was about 6 years old, and I don't remember hearing a rap song before this one ... BUT, I don't remember it being that strange to me. I guess there may have been rap commercials or something before that, but nothing I can remember hearing that would have allowed the song/genre to sound so instantly familiar.