I think he would have actually. I mean, look at Nas' career. The dude's been dropping mediocre stuff since It Was Written (with the exception of Life Is Good), but it was because of his debut and his impact that people hold him in such a high regard.
So, with that said, Eminem would have still been considered one of the best if he had one classic (or two, if you're counting SSLP as well), and have the rest of his catalog be pure garbage. Top five at the least.