Reasonable Doubt is often held up as a prime example of the lag between an album's release and its eventual acclaim as a "classic." Jay Z's debut was not nearly as celebrated upon its release as it would be years later. But even going back to seemingly etched-in-stone classics like Illmatic, you will find that—upon initial release—there was definite dissent.
A classic isn't determined by some objective truth, but rather a consensus of fans, and an aggregation of trends. The "test of time" is something of a myth. What really happens is that a lot of opinions begin to aggregate. Certain narratives form, and begin to win over converts. Every few years, someone will shout "Wait!" And point to a record that, although initially ignored—say, Reasonable Doubt—was, in fact, much better than had been initially remembered. Or perhaps an album held up as a classic (not going to pick on anyone here) wasn't really all that.
But what's happened in hip-hop is that the hip-hop audience has become a lot more cognizent of "classic" status. As a thing to aspire to. As an overarching goal for every rapper, from the lowbrow to the avant garde. Although there are always people calling new records "classics" on a daily basis, as a whole, consensus is considerably more divided than it has been in the past. It's a lot harder to win over the hip-hop nation, because the hip-hop nation is so diverse. Classics are more rare than ever.
The big exception was good Kid, m.A.A.d. City, which spawned a whole new set of arguments, simply because it was the record everyone seemed to agree upon. (This was ours.)