Rakim.
Jay had the better career. He made business and political decisions that enabled him to be at the forefront of the culture and explore different creative lanes. But when it comes down to the mic and rhymes, Rakim is better pound for pound.
Rakim was a pure artist and wasn't geared up to handle the business side that took over the genre, or finesse his way through the cultural changes that came with that. His dominance was based purely on talent. I believe Rakim would've done much more creatively with Jay's cunning than Jay did. They're on opposite ends of the spectrum in terms of handling the evolution of the genre.